Paint Booth

To date I've been priming parts in the garage, and mostly interior surfaces. So if the odd bug or speck of dust got into the final result, I didn't worry about it. But now that we need to prime the exterior I thought I'd better take some precautions to protect the primed surface or face endless hours of sanding!
The mission has turned into one of how to make a portable paint booth without spending any money on it. I had a lot of 1/4 inch and 7/16 bar laying around the shop so I cooked up a modular 10' by 10' by 7' skeleton for a paint booth for starters. I figure I'll use plastic sheeting and duct tape to wall it all in...