Well, your cardboard setup isn't a bad idea, nor is it uncommon for people to choose a cardboard box paint booth. It just lacks ventilation. Here's one I think you would approve of!
http://cs.scaleautomag.com/scacs/for...94/357859.aspx

I just don't recommend using propeller fans. It pulls fumes right into the electrical motor, which is a fire hazard of course. But, this guy used brushless fans, which is a bit safer I suppose. Just not enoughcomputerfans in his cheap set up to really be at top performance. Should have a minimum of 4-6 fans at 50-75 cfm's.