I keep all my loading powders in a wooden cabinet, with a pad lock on it. This is done to keep my grandkids out of it, and for no other reason.
Nothing is going to keep a thief from taking what he wants , if he wants it bad enough. I worry more about the heat and cold difference, along with the humidity causing moisture in my powders than I do about Fires or a thief.
I keep my cabinet in an air conditioned and heated breeze-way, between my house and garage along with all my loading equipment, to keep it dry.
I would think the cabinet should be totally wood, since it would be less likely to draw moisture. As far as exploding powder or loaded ammo, in a fire, I would be far more worried about that can of gas I keep in the garage for the lawn mower, or the solvents in my wood shop, than I would any powder or ammo I had stored, unless you have enough stored to make such an explosion, in which case I would think you have to much stored.