What is the easiest way to do this and fill all of the other holes. Kind of hesitant to weld them shut as that metal is rather thing I'm worried about warping the metal.
+1, if you take your time with the welding and just do a little on each hole until they're filled, you should be ok.
Just turn the temp down a bit on your welder and keep zapping each hole a little at a time. Once the holes are welded shut though, don't kid yourself, you'll have to straighten it (bondo). It's wavy as sht under that spoiler.
I once filled holes on a deck lid with por15 epoxy. Sold the car 2 years later and no issues. It's kind of hack but, I was younger and didn't know any better..