Don---One of the best things I've ever found for making accurate patterns is my $11 glue gun. When I made the pattern for my T roadster floor, I cut a peice of cheap particle board deliberately about 4" undersize from what I knew I would need. My floor was recessed about 6" (the car was channeled that much) so I couldn't just lay the bottom of the car on the plywood and trace around it. With the particle board dummied into place (off the frame), ---actually I propped the body up on four 8" high cement blocks, one on each corner, and propped the particle board up on four 14" high blocks of wood. Then I took a bunch of 12" x 8" peices of cardboard, and set each peice tight against the inside of the fiberglass body, then glued it to the particle board all the way around the inside of the car. When I finished doing that, I lifted the particle board out, with all the cardboard stuck around the edges. I drew a line all the way around the perimeter of the cardboard, 3/8" in from the outside edge, and trimmed it off with my shop scissors. This then gave me the perfect size template to use when I cut my 3/4" marine plywood floor.