Ordinarily, I would mount a 1/4" base of luan plywood just under the roof of the truck and mount the headliner board to that, but with the top chopped, I couldn't really do that because there would be no headroom left. I decided to just use 1/8" PVC foam board, do the design on it, and mount the board in the truck by itself. I hung a piece of chipboard to the roof of the cab with magnets, so I could measure the size of the headliner board from that. It turned out that 48" wide was going to work, and about 36" from front to back. I also planned to bend the foamboard with heat, and use the rear bottom of the vertical part to mount the back of the headliner board to the frame of the truck just above the rear window.
No one lives forever, the trick is creating something that will.