I'd make one from poserboard, or some other semi rigid material. You can relieve the area needed for the carb to pass, mark center lines on it, and mounting hole locations. Then lay it on the firewall and go for it. There are some PDF files floating around the web with the drawing available as well, if you do a search. Still after printing you'll need to confirm the size match before proceeding. I'd prefer to skip all that and just make something....your call though!