If you have, or can find an old, used, refrigerator, or freezer unit that is small, it can make a wonderful powder storage case. Its insulated, and fire resistant. If you must, go ahead and surround it with sheet rock, for additional protection. With the refrigerator, or freezer, you will have to drill a couple of holes, to mount a hasp and lock plate, but then you can put a good lock on the door to keep out the curious. If you dig a hole in the ground for this, you get the benefit of the ground making any explosion go up, and not OUT, for far less damage in the event of an accidental ignition. But do remember to provide drainage for water under the bed so that you don't drown your powder. Remember, that the cans this powder is shipped in is sufficiently safe so as to not worry about heat, or sunlight, or adverse weather setting the powder off. We had a posting here some months ago where a video shows conclusively that static electricity will not set off black powder, so that is not a concern. Just keep it dry and cool, and you should be okay.
You might talk to a local sporting goods store that would like to sell Black Powder, but can't have it on the premises because of restrictions on how close it can be stored to residential dwellings. The owner might be interested in sharing a powder magazine, or bunker that you put on your land, where he can also store powder. He might even help with the expense in building a very adequate bunker on your property and that would take care of all the problems.