You have had many good suggestions. the aircraft industry uses Dollar/Dime patches. apiece of material larger than the hole is put on the back side of the hole and a piece of metal cut to fit in the hole. I have used this very successfully on the firewall of my Model A using JB Weld I held them patches in place with Clecos. Once the JB Weld has set up remove the Clecos and fill the holes with JB Weld. Works great.
