Hi MB, I actually have an ammo box (ex my fathers from his Air Force days in the 40s) but I'm using it for tools at the moment, hence the new box ($15) We were having a discussion about steel boxes at an RC plane club, and a guy said, "I keep all my LiPos in an ammo box for safety". He opened it up and inside was a literal Bomb of LiPos all stacked in there. 6s, 5s, 4s ,-the whole range and full to the top. Everyone said (almost as one) "what if one goes off in there"!!! Holy cr-p!!
Your Idea's a much better one but I think more than one is still risky as the bags obviously aren't foolproof. It would be interesting to test that out with some old lipos to see if nearby LiPos were affected at all. You do need adequate vent holes with a flame suppressor or else you are risking blowing the box up if it's clamped shut. Here's a vid of a tool box test with sand bags, -worked pretty well!. Unfortunately there's no getting away from the smoke!
Also one with an ammo tin which also worked well ( I saw a test on another vid, and the guy said to leave the seal out of the box for venting (the guy in this vid mentions they have done this also)