Eagle: Really nice paint spray booth, I'd never have thought of using computer fans like that, great idea!
Gamera: 20 years ago I used a brushless instrument fan (120AC version) that pumped out a little air. Just the fan and enough ducting to run the paint and fumes out the window. With the newer fans look at the CFM ratings and do a little visualization. The fans I am using are rated at just a hair under 70CFM. I have 2 that I can run on this.
To see if this is enough air, think of it like this: a box 3ft x 3ft x 3ft is 27 cubic feet. A box 3 x 3 x 6 is 54 cubic feet. A box 4 x 4 x 6 is 96 cubic feet. If we consider that I am only using a small airbrush and with both fans going. There is plenty of airflow going out without having to do noise and hurricane. I could probably use large spray cans and never smell it back past about 3 feet away. Of course there might not be quite enough power to push them as much as needed and I might need to get some stronger fans. The main key here is BRUSHLESS . I used an AC (wall outlet) version 20 years ago and it worked great at venting. I know it worked as here is a photo of the air duct. (nasty)
I had no filter over the fan to catch the paint particles at that time. It worked quite well and didn't seem to hurt the fan even though it was somewhat coated with hobby paint too.
I should find out how well the 2 'puter fans work here soon. It has been quite a while since I was doing any airbrushing. So will see.