If you can, run the booth, positive pressure. Grab an old squirrel cage fan . I'd be careful with a box electric fan pulling exhaust... you have a flammable media being pulled out by something electrically driven, think of the possibilities!
Crackedback
You're right about the squirrel cage fan if you can find one easily and find a way to mount it easily,they are usually pretty heavy for the amount of flow you would need. What makes the box fan nice is that it's light and easy to mount in a window or door and seal around it with cardboard and masking tape. Crude I know but safety wise I believe any elect fan motors in the last 10 years are probably induction motors and not brush type with sparks. Positive pressure is also desireable in a well set up booth with a clean rooftop intake which is relatively dust free, the nice part about the drawthrough "garage" booth is that if you use 3 or 4 filters at the bottom of the garage door and soak the area in front of the filters, the velocity of the incoming air is very low and less likely to draw in small insects and such. There are many ways to build a home made booth, this is just the kind I've found to be simple to build and have turned out some really nice paint jobs.
