I store and have stored modern and black powder in a metal box lined with 1/2 inch plywood. Put wheels on it to make moving easy, other than that I keep it near a door to get it out of the house if there would ever be a fire. Do the same with all ammo and anything that goes "bang".
Keep it locked and in a clean dry area.
If you have the room here's a good setup.
I have powders (modern and black) that are probably 30 years old and still work like new.