Someone may come along and correct me, but I believe that steel ammo cans are an acceptable way to store Lipos. I keep old unused working Lipos in one, but I use those fire-resistant storage pouches for my frequently used batteries.
The part where I am unsure is that I understand that when Lipos "cook off", the fire is extremely hot (as in capable of melting/burning through asphalt), so I don't know if that thin ~20 gauge steel is thick enough to not get burned through by a Lipo on fire. In that case, it might not be good enough, but it is still way better than nothing.
Regarding using separators, that is never a bad idea, as redundancy is a good thing in the safety department, but I don't think it is really necessary. The battery bodies touching each other has no effect on anything, and battery plugs are generally made so that there is no exposed metal, which could touch other connectors, cause a short circuit, and a resulting fire. But, imagine something like a paper clip dropping in there and bridging two connectors. In that case, dividers would prevent any potential problems. Extra safe, but mainly for extra assurance, as I don't think it is really necessary (unless you do have exposed metal on your connectors, then separators would be mandatory).
I hope this helps some.