Have carried flat (6-8 sheets 1/2") and propped up on the tailgate (4 sheets of 3/4"). No issues except leaving a bit of red dye stain from markings on the wood edges on the sides of the bed. Depending upon how far you are going and your driving habits, with the plywood laying flat I have gotten away with multiple bungie cords connected from the low rear tie down on one side, around the end of the plywood, connected back to the high rear tie down on the other side. Could also use a ratcheting cargo strap to do the same if going farther or concerned about the bungie cords.