Those lipo bagging their batteries to charge, remember the batteries, especially if older and mildly destabilising, will heat up while charging.
Putting them into the confines of a bag that is fairly sealed will possibly cause them to overheat more and possibly begin the runaway process (heat, smoke, flame out).
I like the idea of a charging area outside and in a specific spot . . .
Like a U shape protecting say bottom and 3 sides, a 'bunker' if you like.
This would allow normal airflow / cooling while charging, allow a lipo fire to flare out and dispense that energy, and relatively safely with gases dispersed to the air.
Putting out a lipo fire is not easy.
In the early stages heat / first smoking, cooling with water or any non alcoholic liquid (soft drinks etc) is supposed to cool them rapidly and alleviate the runaway.
After flaming out, a suitable fire extinguisher is used to smother the flames, then cool the living heck out of them for a good while, even leaving in a bucket of water for days, perhaps later good salty water to really make sure the battery charge is dead for disposal.
Folk here probably know fires in lithium car battery fires take up to 6,000 gallons to put out / cool, and THEN some catch fire in holding yards days after they are towed.