my attempt at a spray booth (knockdown type)

I think that's a GREAT idea, except.......... you might need better support for the plastic sheet. You ARE going to pressurize it and not evacuate it with blower pressure? But either way, you'll be amazed at how far in/ out the plastic will balloon, and you SURE don't want it sucking inwards.

I'm guessing you'll need some sort of "more" support members for the air pressure on the plastic.

The "cheap way" might be to buy, (fer sayin') 1x4 bats, put the plastic on, and screw the bats through the plastic into the piping, then add additional horizontal bats to that??

Just thinkin' outloud.