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I have made a very simple attachment that can support multiple sheets of 4X8 plywood in the back of my Range Rover. I hauled 5 sheets of 3/4" plywood up a steep hill a few days ago.
x2 on front/back ratchet strap. plus the dirty throw blanket on the roof to prevent scratching the paint or sunroofs. and be super careful on bumps with sheetrock, that stuff is brittle and will snap on you.