I just measured a '65 Barracuda original and mine is 1/4" thick Masonite. My 64 Valiant has Masonite slightly thicker than 1/8" with the same style cutouts that Clifcuda 65 posted. Perhaps the formula S cars got a thinner one with the cutout because their tires were bigger....? Here are some photos with dimensions of my original '65 piece.