If I was to do it again -- and I'd sure avoid having to do it again -- I'd go to the store with 1 or 2 sheets of 5/8 or 3/4 inch plywood on the roof rack already. I'd sandwich the thin plywood between either two shets of thick plywood, or a single sheet of plywood on the bottom and some 1x4 or 2x4 boards on top. All held with clamps, and all the clamps roped together so I don't have any flying off into traffic should something come loose.

The aerodynamic properties of plywood can create a LOT of strain on your rooftops carriers. Even with thick, heavy plywood that stays flat, the lift from the air rushing under the panels when you drive at 45, 50 or faster can be enough to rip your cartop carriers off the top of the car, and deposit your plywood somewhere in the path of oncoming traffic.

Drive carefully and take sidestreets as much as possible if you must carry your plywood on top of your car.

Budget, UHaul, and some other rental places rent out pickup trucks for $20 a day plus mileage. If I didn't have my new trailer I'd consider getting enough wood on a trip to the lumberyard to make renting a truck a reasonable proposition. Carry some weights with you, as thin plywood panels CAN blow out of the back of a pickup if they are not well secured, or weighted down.

PGJ