If the metal ammo can doesn't work unmodified, it most certainly would with the lining I mentioned in my earlier response, and I would very much like my insulation method to be tested, but have no dead LiPos, nor do I want to make another box just to test one. I'm fortunate enough that the floor of my blacksmith shop is dirt, and have no need to worry about my batteries taking anything with them.
Another thought I've had, is that with the lid sealed well enough, I would think that a LiPo would run out of oxygen to react with. Not before damaging a plastic box, but maybe before the box gets hot enough to melt.
One of the people that frequents a local track had a LiPo go off in a metal ammo can, it barely left a skid on the neighboring battery, barely leaked any gasses into his garage. He opened the box in the spring to use a battery, and found his surprise, barely remembering that day a month before when he thought he smelled something electronic burned.
I don't like the idea of having a bare metal (paint doesn't count in this case) interior for battery storage, it's conductive to both heat and electricity, and the plastic on a battery failing from heat is what helps a LiPo's thermal runaway be so spectacular at times. (insulation goes out, and things start shorting)