I did some looking around today and some napkin sketches, then stopped by Lowes on the way home for pieces.
90 minutes of work in the garage later, and this is the result:
The inside dimensions are 48" wide and 18" high. The floor is 30" deep and the top is 18". Six eyes on the top will let me hang pieces, including barrel assemblies, for easy coating. The box is big enough to hold nearly any rifle in one piece, and any shotgun with the barrel removed.
The fan is a $15 bathroom fan rated for 50cfm. A furnace filter will keep overspray out of the fan motor. I clipped the outlet off an extension cord and connected the fan to that. A length of flexible duct (not shown) will be run to a board that I can put in a window.
I need to add lighting of some kind, but for the immediate future intend to use a clamp-on or hanging work light.
1 sheet 1/4" plywood, cut into one 30" and three 18" sheets
3 2x2x8s, lengths will have to be measured based on your own assembly
1 box of 1-1/4" drywall screws
1 bathroom fan
1 8' grounded extension cord
1 furnace filter
1 length of flexible duct
All told I think I have about $70 and two hours in. Piece of cake, and it should suit my needs beautifully. If anyone wants more detailed info, drop me a note.
Disclaimer: this isn't a proper spray booth. The fan isn't rated for hazardous/flammable materials. I'm trusting that the quantities I'll be using are so small that it won't matter. If you're doing this regularly, for-profit, or in larger quantities, consider spending the money on a correct assembly.