I have built five or six, and finally have one that I really like.
I started by using cardboard boxes with a rectangular opening cut in the top. I cut a piece of thin plywood just a bit larger than the opening, which was the size of a rectangular auto air filter. I cut a four inch hole in the plywood to match a muffin fan I had bought. Screwed fan on top of plywood, duct-taped the filter to the bottom and dropped it over the hole.
It worked, kinda, but the fan was not powerful enough. I found you need at least 100 cfm. Bought bigger muffin fans. These plastic cased fans are available in quite a range of airflow, are brushless (no sparks), and not that expensive. I buy them from electronics parts suppliers like Jameco- they are used to cool electronics. I buy the 110V AC models so I can just solder on a cord and plug it in.
Decided I wanted a permanent cabinet- no more cardboard. Made one from particle/fiberboard, around a 16 x 20 furnace filter- less pressure drop and cheaper filter, widely available at hardware and home improvement stores. Worked fine, but very heavy when it came time to change filters. I have it hanging from two hooks on wall, so have to take it off hooks and put it on bench to change filters.
Latest model, a keeper, is much lighter. I made a frame from 3/4 x 3/4 pine (with a few wider pieces), and covered frame with 1/4 inch plywood. Also made a hinged back that made changing filters much easier. I think I have a photo of it somewhere on photobucket- if I can find it I will post photo. This one has a 125 cfm 5 inch muffin fan.
By the way, there have been several threads on homemade spraybooths in the Tools, Techniques and Reference Materials area of the forum- be sure to check out that area.