The best fire safe I have ever seen was a refrigerator freezer.
I have seen then burned up on the outside but the food was still intact on the inside.

One man I know, bought a chest type deep freezer, lined the bottom with collectible coins, put in an inch of water, froze that and repeated the action the next day,,and the next,,etc, until the freezer was full of his collectible coins, full of ice, impervious to theft and fire.

That's my $0.02 and I'm sticking to it!!!!!