A wooden box, or even a styrofoam cooler is a much better container for storing black powder than a gun safe. You want the powder to burn, if it gets above the flash point of the powder, NOT EXPLODE.
I know of one friend who has a large wooden box, with spacers made of wood to separate each can from the other. The Bottom and sides of the box are made of thicker wood than the top. He figured that since heat rises, he would be better off it the powder burns going straight up, rather burning out or EXPLODING to the sides and setting off neighboring cans. He keeps his box in a basement where its cools, and the last place flames will get to. But, he used to live in a house without a basement, and he kept the box in the back of his garage, where it was both cool and dry.
The first year he stored powder in the box, he checked the air temperature in the box on the hottest days. When it 100 degrees F. outside, the temperature in the box rarely got up to 75 degrees. Since the flash point of BP is generally 450 degrees, he felt it was safer storing black powder in his garage than storing gasoline for his lawn mower! I store, and still recommend storing powder in the cardboard shipping box it comes in. The Company really does know how to store the powder during shipping. And, from personal experience, I can assure you that the back of a Semi-Trailer truck driving down the highway in the summer heat gets a lot hotter- up to 140 degrees!--- Than what occurs in a garage, or in your dry basement.
The idea of sinking 5 gal. buckets into a hole in the ground, under a storage shed also makes sense. The shed keeps the sun from heating the container, while the soil keeps the ambient temperature below 70 degrees. The same idea applies to Cisterns, where the stench may be off-putting, but the temperature is often in the 60s, even in the summer time.
I personally like styrofoam coolers because they are so cheap and light weight.
I keep my eye out for those old, metal, Coleman coolers and ice chests that were all we had up through the 1960s. You can still buy them new, but I am hunting the garage and estate sales for a " deal". For storing BP, a used cooler with some rust on it, that I can buy cheap is perfect. I can spray paint the rusted area with a Krylon paint to stop the rust. I am not interested in winning fashion points with my cooler at a picnic! :blah: :grin: You can also put a latch, and hasp to hold a lock on such coolers, and they make a pretty good "seat" when needed.
I tend to use the " My GARBAGE" technique for home security. Things of value are stored under a lot of "stuff" that isn't. If the " stuff " looks enough like clutter, or Garbage, and too bothersome to move, most burglars and thieves pass it by. A flat top old cooler in the garage with a stack of junk piled on top of it isn't going to get a second look. I interviewed way too many thieves and burglars, and then their victims( who told me what my clients missed of value, and where it was located) to believe otherwise.