I migged a lot of factory holes as part of my engine bay cleanup
I used a bag of mixed size iron flat head nails.
Hold them in place with the head filling the open hole, mig till solid, then grind off the stem.
I did a few at a time before grinding and I had to laugh at one stage cause my engine bay looked like the guy from Hellraiser who had nails coming out of his head...
it does the job quick and easily.