Use one of these
to clean up and size the holes and one of these
to punch some 18ga sheet metal disks to fit snugly in the holes. Place the discs in the holes and gently tap them with a hammer and dolly so they are snug. Use a mig welder to weld the discs in solid. Use very small tack welds and let each weld cool before doing the next tack. Keep this up until the entire circumference of each disc is welded solid. Grind down the welds and if done carefully, there should be no warpage.