A simple way to do it is to print out the firewall template (from the plans) on paper, tape that onto 1/8” plywood, and then use an awl to mark the location of the six holes. Just treat the square hole as another round hole to make it easier. Then trace the perimeter. Lastly use a drill to drill out all the holes and cut the sides using a small saw, or even a utility knife. 3/32” plywood is better than 1/8” but it’s difficult to find, but I’ve found some of the decorative wood craft boards at DollarTree are about 3/32.