Charging lipos in a closed container can be dicey. The best you can hope for is to contain or isolate a fire if it happens. Many people who use ammo boxes for lipos remove the air tight gasket to relieve the pressure in case a lipo "goes off". Some even drill holes in the can but I'm not sure how well it would work. Check youtube for lipo fires or lipo explosion for their behavior.
I currently have my lipos in lipo sacks and in a file sized fire safe. The back hinge of the safe is busted so in theory if a lipo vents the hinge side should raise enough to relieve the pressure. If I didn't have that safe I would probably construct a bunker from a couple of 16x8x2 (or 4) concrete blocks for the top and bottom and 1 or 2 16x8x8 concrete blocks with the rib/s busted out. I've also read about guy who did this but on top of the block he put a plastic bag of sand thinking that the fire would melt the bag and release the sand down on the lipos extinguishing the fire.
Rcgroups battery and charger forum has this question asked a couple times a week if you want to read more.