I've completed my knock down spray booth based loosely on Highland Hardware's plans. It is mostly built from materials I had around, with the exception of 3 sheets of Luan I had to buy for the top and sides. The fan is a 20" "8800 CFM" fan I found at Harbor Freight.
It seems to work well in tests with water in my spray gun. I just sprayed water in the air for a couple minutes and did get the floor damp "inside" the booth, but not outside The filters seemed to be wetter than the floor.