Never done it myself but I've discussed building one with a few people who have knocked up temporary booths to paint aircraft, popular method is a light frame, either one of those temporary car ports or timber say 19mm square, just enough to stop the 'walls' blowing around. Drop sheet poly is popular as it lets the light in. You're not clear how you intend to enclose from the joists, are you just dropping to the floor or are you covering in the roof ? That's where all the contaminants will come in.
I'm guessing from your mention of swivels you intend to stand outside and spin the object in front of the gun ? 6' square sounds a little cramped if you intend walking around the project.
Try googling 'experimental aircraft spray booth', bound to be a few people who have posted their solutions online.
Last edited by Mdawson; 06-12-2014 at 08:59 AM.