Technically, you need at least an indoor type 4 magazine.
Go Here:
http://www.access.gpo.gov/cgi-bin/cfrassemble.cgi?title=200327In the search terms window enter "powder magazine"
Open up the .txt document in the resulting list that says "Subpart K - Storage" for all the magazine info.
For an indoor type 4, here is what they say:
(b) Indoor magazine--(1) General. Indoor magazines are to be fire-
resistant and theft-resistant. They need not be weather-resistant if the
buildings in which they are stored provide protection from the weather.
No indoor magazine is to be located in a residence or dwelling. The
indoor storage of low explosives must not exceed a quantity of 50
pounds. More than one indoor magazine may be located in the same
building if the total quantity of explosive materials stored does not
exceed 50 pounds. Detonators that will not mass detonate must be stored
in a separate magazine and the total number of electric detonators must
not exceed 5,000.
(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.
(3) Hinges and hasps. Hinges and hasps are to be attached to doors
by welding, riveting, or bolting (nuts on inside of door). Hinges and
hasps must be installed so that they cannot be removed when the doors
are closed and locked.
(4) Locks. Each door is to be equipped with (i) two mortise locks;
(ii) two padlocks fastened in separate hasps and staples; (iii) a
combination of a mortise lock and padlock; (iv) a mortise lock that
requires two keys to open; or (v) a three-point lock. Padlocks must have
at least five tumblers and a case-hardened shackle of at least \3/8\
inch diameter. Padlocks must be protected with not less than \1/4\ inch
steel hoods constructed so as to prevent sawing or lever action on the
locks, hasps, and staples. Indoor magazines located in secure rooms that
are locked as provided in this subparagraph may have each door locked
with one steel padlock (which need not be protected by a steel hood)
having at least five tumblers and a case-hardened shackle of at least
\3/8\ inch diameter, if the door hinges and lock hasp are securely
fastened to the magazine. These requirements do not apply to magazine
doors that are adequately secured on the inside by means of a bolt,
lock, or bar that cannot be actuated from the outside.
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Note that they don't tell you about thickness of walls. I recall reading someplace that the National Fire Protection Act wants 1 inch minimum wooden walls.
Also, look at the document entitiled "Sec. 555.218 Table of distances for storage of explosive materials" to see where you can keep your magazine.
Note that you don't have to get your magazine approved by the ATF (local approval may be required) if you are not a dealer or manufacturer (just a user of under 50#), but they expect you to meet the construction rules anyway.
As a side note, local fire department rules may differ from, and even conflict with the ATF requirements. In one town I used to live in, they required that powder be kept in surplus ammo boxes. That conflicts with what the ATF wants, as there was no wood under the metal (the wood slows down internal temperature rise if there is a fire). So, to be 100% legal, you have to check local requirements and try to meet both them and the ATF's
