http://www.alliantpowder.com/getting_started/safety/storage_handling.aspx
Also, there is this:
Build a box of 1" wood. Cover it with 5/8" sheetrock. Make a cover of, from inside out, 3/4" wood, 1/4" plywood, topped by 5/8" sheetrock. The lid/door should seal but not be so snug that it won't open easily. If the gunpowder does ignite, it will pop the lid off and allow the remaining powder to burn rather than explode.