I think your original idea is on the right track. When I made templates for the glass on my chop I used some 1/8" plywood but you could also use masonite or anything very rigid. Cardboard won't work. Also have your gasket material on hand so you know exactly how much room you need. I did a v-butt windshield so I had some extra length of gasket that I cut into shorter pieces to space around the opening. The same with the rear opening. I actually had the rear and side glass cut and tempered instead of the original laminated. Just my choice.
