I built one out of the panels of windows I replaced in the house. They're probably 2.5 ft by 3.5 ft or so. Spent $8 on some brass pin latches so I can set up tear down fast. I set it on top the plastic dog house base that never got used (dang house mutt...), with either a glass or plywood top and a used furnace filter to keep the stuff inside. Use your junk, get creative.
For the bigger stuff, I fould a guy who will do stuff well and cheap. My hours in the shop are too few and far between to spend all day saving $30 to do a fender. He loves doing it and is cost effective. Shop around & ask around!!!!
