I built my own about a year ago using a bothroom fan of about 55cfm and the booth was built large enough to house a 1/48 B-17, so its got some room. This fan has no problem keeping the air clear. I built the whole thing using small sheets of 1/8" board. (Don't know if its partical board of what, but its down in the lumber section w/ the pegboards and all.) I also used a piece of peg board for the bottom. All the frame work was done with 1x2 boards. I used 1/16" plexiglass w/ hinges for the front. For the exhaust, I used a dryer vent hose and exterior vent. Drilled a hole into a spare piece of the partical board that was as wide as my window and attached everything together. To use, I use open a window, place the "ventor", and then close the close window on the board. Vents out like a charm. Oh, I also added 3x bar-type lights on the inside for lighting.
Total, I think I spent only like $80. Much better than the $200 some people ask for a small metal one.