I just don't see how a tubular frame would work in the confines of a M72 cockpit. Using the ideas I already talked about, I would take a piece of scrap 1/4" plywood about 6" wide and whatever length needed to go from the floor of the cockpit to the top of the helmet and slide that vertically in behind the seatback. His weight against the seatback will keep it in place along with the shoulder harness. Then I'd make a helmet-sized U-shaped bracket out of PVC pipe (3 straight sections connected by two elbows) and through-bolt that to the plywood. Use some pipe insulation (or a $1.00 "swimmie" from the Dollar Store) on the PVC so it'll grip the helmet without vibrating and still let him move his head around a bit. Does that make sense?