Painting dilemma (long). What would you do?

I'm actually thinking of building a frame outside and adjacent to the barn, of PVC or maybe light angle iron. It only has to hold up the plastic and maybe a little wind loading. The area in the building is rather cramped on the sides (and spraying the rear quarters will probably be the most difficult), but outdoors I can make it as wide as I want.

After looking at several threads on DIY booths, it looks like the intake should be up high and the exhaust at the floor (downdraft). Many have found that two box fans (one in, one out) are not sufficient to clear the paint mist. I want a positive pressure inside so that the plastic does not have to be perfectly sealed to keep pollen and gnats out, too.

I'm still hoping the guy who said he'd do it will quit waffling. I am supposed to see an example of a similar priced and detailed job this week - but will not be surprised if he backs out. Wish he'd commit or not so I can start building the booth!