Congrats on getting a down bag!
Plastic bags are only used for shipping, not storing.
It’s great you have an a/c storage unit.
A cotton cover sheet sounds like a good idea. Cotton promotes breathing, and keeps the dust off.

My Kelty bags hang from the ceiling since they have hang loops.
My Western Mountaineering bags are in a large cotton bag which also hangs from the ceiling.
I like the hang loop idea but I don’t know if one way is better than the other way (large cotton storage sack).

-Barry