Some good idea's here....... this is what I would do, make a patch for it, after it it cleaned up and use some panel bond, with may be some pop rivet's to hold it in place. Personally....... I don't like welding with mig in this position, as no matter what you do to protect yourself from spark's and other, you alway's seem to get burned up. Been there, done that!! My brother burn't half of his wig off, several year's ago, mig welding underneath of a car on a single post lift, when a big spark landed in his long hair, damn good thing he had a hat on, as I think it was winter time.LOL!! So, whatever you decide to do, be careful under there, and good luck with your fix!