Normally, black powder spilled on the ground and lit will just burn like a fuse. However, I have it detonate more than once. Its funny stuff, and it can go off on you. A friend's powder horn plug fell out as he was walking in the woods, dumping most of the 3Fg powder in it on the ground. For safety, he asked me to light it. I put a small charge of powder down my barrel and pointed the barrel at the powder and fired the black change. His powder detonated, in a loud bang, much smoke and a rising cloud going up and through the trees. It made him jump and fall, and his horn cam apart again, dumping what powder he had left. So, being brave souls, we lit that off, too. This time it just burned! My ears were ringing from the first explosion for a day or two. A good steel box will direct the explosion upward, but you will still have an explosion. I think an old freezer chest, or old refrigerator, laid on its side, so the door swing upward, wwith a hasp and lock is the easiest, and cheapest way to have cool dry storage. Just keep the sun off it, and consider burying it in a hole in the ground, so you get the added protection of the earth, and the cool temperatures of the earth, to help keep the powder dry and cool. Because of corrosion problems with earth, I would spend the time and money to pour a concrete foundation, and use bricks or cinder blocks on the sides to hold back dirt and water. Then put 2 bys down to keep the box out of direct contact with the cement. That way, you could also drill a couple of holes near the top edge of the box to allow air to go in and out of the refrigerator or freezer box so that in the event that kids get into it they won't suffocate before you find them.