If you remove the rubber seal, you should be fine. The point is to allow the gases to vent. If you don't, a sealed ammo can will build pressure until it explodes and possibly sends metal shrapnel everywhere. You want to vent the gas/pressure while minimizing the possibility of flames igniting nearby materials.
This is how I store mine. I got an ammo can from harbor freight. Removed the seal on the lid. When charging I just sit the battery down next to my charger and let it do it’s thing.