The correct way is to cut sheet metal into little disk "plugs" that fit snug. If you have tig you can easily control the heat or with mig just make little spot welds all the way around. Get a piece of cold rolled sheet metal or scrap and drill some holes then practice filling them this way. Get your welder settings figured out on the tests.
No need to remove any sheet metal, just protect your parts from weld spatter. Especially cover the glass! Take your time and cool the welds with a damp rag as you go if it gets too warm. Be sure to prep to bare metal around the holes first of course. A little wipe with body filler, finish out and your done! Be sure to finish or seam seal and paint on the outside too as otherwise water will migrate into your bondo and blister up on the inside.
This is a 15 min fix, I have been building custom cars and a professional paint and body man since the late 70's and have done more of these than I can remember...
That said, on my personal car that is not too pretty (a cobblers child wears no shoes) 😁 I would probably just throw a bolt in it. If my car was finished I would repair it correctly.
Last edited by Matter; 09-12-2019 at 11:34 AM.