RE: H9 P-51 Rebuild help please
Get some aircraft grade plywood, about 3/8" or so, trace out the outline of the firewall, cut it slightly large, then sand to fit. Glue it in with epoxy and triangle stock. I've done this on my first few trainers. You should be able to find someone else with a PTS mustang, or as was suggested, go to the hobby shop and see if they will help you. Good luck