It's probably easier to tell you what you shouldn't use for a spray booth as opposed to tell you what can be used. Most anything fibrous or of a natural material (old plywood) isn't the best due to the fact that they tend to "shed" when you are coating. It's not that I havn't used them or we still don't make them out of that material.... it's just easier to avoid pitfalls of things in your coating if you stay away from them. If you do use plywood.... here are my suggestions. Get a nice thick latex house paint and slather the inside of your booth with it. This will seal it from any stray splinters knocked off over time, make it easier for cleanup ( pinhole free coating so nothing can stick to it) and more than likely you'll be painitng it white... so you'll be able to see what you are doing easier in there. One thing that is a MUST (and I've talked to some of you on the phone about this VERY thing!). You have to have some sort of sacrificial metal in the back of the booth to catch stray ionization. Take anything ( an aluminum plate, if you will) and put it past the part you are coating and GROUND it so it catches the stray ions from your charge. Your parts will not only thank you for it from a smoother coating... but you won't build up and voltage and possibly risk a healthy shock when you go to clean the booth. It's bad enough that when you use a shop-vac to get the powder out it builds up a charge from friction in the suction tube (this is a very loose way that tribo charging can happen... more on that in another post), but to have nowhere for the stray charge to go is gonna be surprising to you when it's YOU that it goes to! lol.
I've seen booths made out of sheetmetal (my preferred choice), lexan or plexiglass, Wood frames with plexi panels, all plywood.... even cardboard. There is no wrong way for the hobbyist to make a booth unless you spent a fortune on it or didn't adhere to the basic rules as stated above
Hope that helps.... Russ