Long story short, "Water Proofing" would be nearly impossible with the current mavic design... Without significant major modifications (And major voiding of warranty), just don't fly in the rain unless you have an extra thousand bucks laying around to buy another Mavic
But if you really want to go down that path for the crazy of it
then...
You could make it more "water resistant" (especially from downward falling rain) with a few considerations:
- Right now the MP gimbal is likely sensitive to water. This would be difficult to properly protect from rain
- Behind the gimbal is an air intake. This has fans actively sucking in air (and water mist/vapor in rain) into the electronics compartment.
- The SD and USB covers are not water tight, some way would need to be devised to protect them better
- But the good news is the top shell of the mavic is mostly solid. Would need to seal up openeings for sensors (vision etc) and hinges for arms
- Flexible silicone would likely work for some of this possibly.
- The big challenge is the battery compartment.
Now that I'm thinking of it, the best you could realistically achieve is silicone the entry points/joints in the arms. And add a silicone "rain coat" that slips over the back of the body including battery compartment and makes a single seamless surface that can protect it from downward falling rain.
Ultimately you're still going to suck water through the vents though...
Oh, and the motors can't really be protected. That said brushless motors are actually fine to get wet (as long as the water isn't full of electrolytes). But you could make the motors themselves 100% waterproof by potting them (check out DIY Subsea ROV forums, they use brushless motors openly exposed to seawater, which is highly conductive, but with the proper prep the motors are just fine)
Just to confirm, I don't recommend doing this at all... Just don't fly in the rain
(But if you do decide to go do it, take pictures and video and share! lol)