I used 1/8" ply and cut a piece the same size as the spinner, tack glued it onto the back of the spinner with 1/8 inch spacers. Then I built a frame at the firewall using 1/4" balsa sticks and I used some more of the sticks to join the frame to the front plate. I then used 1/4" balsa for the sides and top and put triangle stock in the corners. Then I sanded it to contour and made a 1/16" plywood bottom that is removeable. The whole cowl is screwed to the firewall through the back frame.