Hi.
I've lived on only solar power in my RV for 4.5 years, without hookups or a generator.
I hike, backpack, etc. I'm DIY and I'm cheap. And I have a Mavic Air. I think I can help with this.
I have carried many ridiculous things on backpacking trips. If you have the will, your legs will carry your stuff. Let's not agonize over weight.
Charging a powerbank from solar and then going from that (either a 12V or 5V output) to charge the Mavic is not an attractive option. The charging will be slow and you'll lose a lot of energy to redundant voltage conversions. You already have the batteries, why add one? I think you should go directly from the panel to the battery.
If you do that, you have to be sure the charge will stop when the batteries are full. Because fire. Normally this is the job of the charger, so I would advise buying the 12V mobile charger and using that. However, these batteries do have intelligence built in (such as self-storage mode), so perhaps that circuit is in the battery. But the batts likely take for granted that a DJI charger will be used so may not safely handle anything much higher than 15V input. "12v" Solar panels generally make about 18V. So I still think you should use the 12V charger. You need the connectors anyway; it's worthwhile just for that. Look for a voltage regulator ("buck converter") online with a variable output adjustment. Amazon has these. Make sure it can handle >20V input and is rated for at least 10A. The low-cost ones are generally not capable of handling much power (for small electronics projects) so this may take some hunting to find -- look for large heat sinks on the PCB. This will regulate the power coming from the solar panel (which will be at varying voltages and currents depending on conditions) down to say 14V so the mobile charger can work correctly. This won't be the most efficient arrangement, but is simple and low-cost. Connect a female cigarette-lighter socket to the output of the buck converter, then test that it's outputting correctly (set it to 14.0V output) with a multimeter before connecting the mobile charger. A fuse and on/off switch on the input side is a good idea too, so you're not connecting things together under load (solar panels are always "on" if there's sun). You can use a fuse as a primitive on/off switch by simply pulling it...just don't lose it.
Specs say the Pro2 battery is almost 60Wh capacity. That may sound like 3 hours of charging with a 20W panel, but that's only in perfect conditions (which never happen), and doesn't account for charging inefficiencies. It also doesn't account for the power loss due to voltage regulation. A 20W panel that is outputting 18V but is bucked down to 14V will effectively only deliver 14 / 18 * 20 = 15.5W. If you stop walking at 10am and aim your 20W panel directly at the sun, best case you could maybe charge one battery to full by 2pm (4 hours * 15.5W = 62Wh). If you plan to strap the panels to the top of your pack (do put them on top, not the back!) and charge while you walk, count on taking basically all day for one battery with a 20W panel (clear sky, no haze). It may even take a bit longer than that. Make sure you're not casting a shadow on your panel with your hat!!! (use the drone to take an overhead video of you walking with your loaded pack and panel on top). Even small shadows kill solar output.
So, if you will burn more than one battery per day, you'll need more panel. That's likely too bulky for practical use. AIMS makes a rather efficient 60W foldable panel for about $200, but it's about the size of a movie poster. This seems impractical to me for packing, attaching to pack, and dealing with wind and other trail conditions. If you can find 30W of panels that actually fit on your pack, that's ideal, but I suspect 20W is about as big as practical. One flight per day ain't that bad, just plan it and make it count!
A lot of the 20W folders I see have USB outputs. USB is at 5V and this means the panel has a regulator built in already. If you cut out the regulator, AND if the panel is configured for typical 12V output, you could use it. But it's also possible the panels are configured to output "6V" (to make the regulator cheaper and more efficient). I think this won't work well for you, because at low voltages you'll only get usable current in very, very good light. Since you can't know this until you cut up the panel, I'd definitely look for one that does NOT have a USB output already on it.
Keep in mind this is all just charging the bird. To charge the controller, use a cheap 12V USB converter in the receptacle from the solar panel -- but realize this will take time away from charging the Mavic's battery. Use this in early AM or late PM when panel voltage is too low to meet the cutoff of the charger (panel voltage tapers to 0 as light intensity reduces). It may be worth adding a simple voltage readout to your setup (on the input side of the voltage regulator) so you know when this is happening. Some regulators have them integrated already.
You'll need some kind of plastic case to mount the regulator in, because it will have lots of sharp bits and needs protection from shorting the PCB. Don't seal the heat sinks inside without leaving airflow. It won't be waterproof, but you won't be using it in the rain, anyway : ).
I think this can work, but it will be a bit of a stretch. Leave yourself time to test and experiment at home before setting out!
I've lived on only solar power in my RV for 4.5 years, without hookups or a generator.
I hike, backpack, etc. I'm DIY and I'm cheap. And I have a Mavic Air. I think I can help with this.
I have carried many ridiculous things on backpacking trips. If you have the will, your legs will carry your stuff. Let's not agonize over weight.
Charging a powerbank from solar and then going from that (either a 12V or 5V output) to charge the Mavic is not an attractive option. The charging will be slow and you'll lose a lot of energy to redundant voltage conversions. You already have the batteries, why add one? I think you should go directly from the panel to the battery.
If you do that, you have to be sure the charge will stop when the batteries are full. Because fire. Normally this is the job of the charger, so I would advise buying the 12V mobile charger and using that. However, these batteries do have intelligence built in (such as self-storage mode), so perhaps that circuit is in the battery. But the batts likely take for granted that a DJI charger will be used so may not safely handle anything much higher than 15V input. "12v" Solar panels generally make about 18V. So I still think you should use the 12V charger. You need the connectors anyway; it's worthwhile just for that. Look for a voltage regulator ("buck converter") online with a variable output adjustment. Amazon has these. Make sure it can handle >20V input and is rated for at least 10A. The low-cost ones are generally not capable of handling much power (for small electronics projects) so this may take some hunting to find -- look for large heat sinks on the PCB. This will regulate the power coming from the solar panel (which will be at varying voltages and currents depending on conditions) down to say 14V so the mobile charger can work correctly. This won't be the most efficient arrangement, but is simple and low-cost. Connect a female cigarette-lighter socket to the output of the buck converter, then test that it's outputting correctly (set it to 14.0V output) with a multimeter before connecting the mobile charger. A fuse and on/off switch on the input side is a good idea too, so you're not connecting things together under load (solar panels are always "on" if there's sun). You can use a fuse as a primitive on/off switch by simply pulling it...just don't lose it.
Specs say the Pro2 battery is almost 60Wh capacity. That may sound like 3 hours of charging with a 20W panel, but that's only in perfect conditions (which never happen), and doesn't account for charging inefficiencies. It also doesn't account for the power loss due to voltage regulation. A 20W panel that is outputting 18V but is bucked down to 14V will effectively only deliver 14 / 18 * 20 = 15.5W. If you stop walking at 10am and aim your 20W panel directly at the sun, best case you could maybe charge one battery to full by 2pm (4 hours * 15.5W = 62Wh). If you plan to strap the panels to the top of your pack (do put them on top, not the back!) and charge while you walk, count on taking basically all day for one battery with a 20W panel (clear sky, no haze). It may even take a bit longer than that. Make sure you're not casting a shadow on your panel with your hat!!! (use the drone to take an overhead video of you walking with your loaded pack and panel on top). Even small shadows kill solar output.
So, if you will burn more than one battery per day, you'll need more panel. That's likely too bulky for practical use. AIMS makes a rather efficient 60W foldable panel for about $200, but it's about the size of a movie poster. This seems impractical to me for packing, attaching to pack, and dealing with wind and other trail conditions. If you can find 30W of panels that actually fit on your pack, that's ideal, but I suspect 20W is about as big as practical. One flight per day ain't that bad, just plan it and make it count!
A lot of the 20W folders I see have USB outputs. USB is at 5V and this means the panel has a regulator built in already. If you cut out the regulator, AND if the panel is configured for typical 12V output, you could use it. But it's also possible the panels are configured to output "6V" (to make the regulator cheaper and more efficient). I think this won't work well for you, because at low voltages you'll only get usable current in very, very good light. Since you can't know this until you cut up the panel, I'd definitely look for one that does NOT have a USB output already on it.
Keep in mind this is all just charging the bird. To charge the controller, use a cheap 12V USB converter in the receptacle from the solar panel -- but realize this will take time away from charging the Mavic's battery. Use this in early AM or late PM when panel voltage is too low to meet the cutoff of the charger (panel voltage tapers to 0 as light intensity reduces). It may be worth adding a simple voltage readout to your setup (on the input side of the voltage regulator) so you know when this is happening. Some regulators have them integrated already.
You'll need some kind of plastic case to mount the regulator in, because it will have lots of sharp bits and needs protection from shorting the PCB. Don't seal the heat sinks inside without leaving airflow. It won't be waterproof, but you won't be using it in the rain, anyway : ).
I think this can work, but it will be a bit of a stretch. Leave yourself time to test and experiment at home before setting out!