Here's a tip from the fire codes. Dry wall is fire rated, based on how thick it is. You can take an ammo can, and simply hot glue, or epoxy glue on dry wall on the sides, top and bottom, to give the can more insulation from fire. Glue a 1/2 in layer of styrofoam on top of the first layer of drywall, and then a second layer of drywall, making more than an inch of material between the fire and the ammo box, and its very likely that the fire will be put out before anything inside the can gets hot enough to ignite. Go ahead and use joint tape and compound to seal the corners and edges of the drywall. Its good practice for doing repair work at home. Make allowances for the hinges, or change how the can is opened. That thermal mass of sheet rock( dry wall) and styrofoam, will also act as a heat sink in case something falls and pierces the dry wall, taking the heat away from the contents of the can. You can even glue aluminum foil over the first layer of sheet rock, and under the styrofoam to provide insulation and a better heat sink. heat sink.
I recommend using a styrofoam plastic picnic cooler, because they are inexpensive, insulated, and come in a large enough size that you can easily pack many pounds of powder and other supplies, along with descicants to keep them dry. The old Coleman coolers were trimmed in metal, and often came with locks, or hasps. They can be easily pad locked, and make great storage containers for powder. Going up in size, you can use an old refrigerator. Again they are insulated, well, and you can add insulation as I have described, to the outside. To protect the contents from even intense heat. A call to your fire department will get the fire ratings for various thickness of dry wall. Old Refrigerators, being metal, are easy to padlock. The motor and refrigerant don't actually have to be operational to keep your powder dry and cool. where you put your powder supplies will depend on whether you can remove them in the case of an actual fire. You will have to decide that based on your floor plans, and a balance between the need to secure the powder from thieves, and the need to be able to remove it in case of a fire. An old root cellar under the house has its appeal, too.