For Type IV indoor magazines, the BATFE regulations state:
27 CFR 555.210(b)(2) Construction. Indoor magazines are to be constructed of masonry,
metal-covered wood, fabricated metal, or a combination of these
materials. The walls and floors are to be constructed of, or covered
with, a nonsparking material. The doors must be metal or solid wood
covered with metal.
This leaves a lot of room for interpretation by field inspectors on what is meant by covered with a nonsparking material. The explosives industry would like to get this requirement changed because almost all explosives used these days are not spark sensitive. The BATFE argues that because a spark sensitive material (BP being about the only one) COULD be stored in the magazine, it must meet the nonsparking requirements.
I recent;y purchased the magazine offered by MissileWorks. It is large enough so that larger single use and assembled motors will fit, comes with plywood lining, and has the NFPA 1127 required verbage on the top.