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1 X 1 square tube, about .060. And all the joins are 1.250 X 1.250 square, so that the 1X1 slips inside and bolts. To make the booth bigger or smaller, just add or remove joins and lengths.
Exhaust is an old house furnace fan for the exhaust, blowing through a 10" concrete pile tube. I've got an old slab door with a hole cut in it that the 10" tube slips through. So I open the walk in door, and set the old slab door in the opening. Secure and slip the tube out. Hey, it's all home project stuff and it works!