Get two pieces of metal one that is used to patch the hole and a strip that will act as a backer.
It doesn’t need to be copper. Just Any metal.... same as the patch.
Cut the backer metal strip in such a way to be able to slip it behind the hole but longer than the hole in one direction.
It can be just a narrow strip.
Drill a hole in the center Of the strip and thread a thick wire through the hole and knot it to the back.
Wet the insulation in back (it will dry)
Slip the metal strip into the hole so the longer sides catch the Edge of the hole.
Hold it with the wire.
Tack weld it in place.
Cut the wire off.
Now you have a backing to place your patch against to fill your hole.

To make the patch, place masking tape over the hole.
Cut along the hole edges and carefully Remove the cutout.
Place the cutout on your patch metal and cut out the patch.
You should have a perfect fit.

For small holes, you can weld them closed using the same backing metal. You don’t need copper.


Mike

1940 Chevy 1/2 Ton presently... Almost done
1940 Chevy Business Coupe... In pieces