Lay a 72 inch board flat and snap your chalk line across it diagonally from corner to corner. Repeat this process on the other board.
Cut along the lines on the two boards to divide them into four equal size triangular pieces.
Set the boards on their 72 inch edges so they are parallel, 14 inches apart and so their hypotenuses are all in alignment.
Lay your plywood sheet on top of the boards so the 72 inch edges are flush with the outside surfaces of the two outside boards. Screw the plywood to the boards using 6 screws for each board. Space the screws 10 inches apart, leaving the two outside screws 6 inches away from the 48 inch edge of the plywood.