Use the smallest wire you can and set the welder on low, and be real quick with it (don’t hold the welding in one spot for more then half a second).
I weld panels in cars with my welder with no gas all the time, but it’s very very hard without gas and takes practice, just do little tacks at a time and work your way around the hole and you should be ok.
Or you could always just use body filler ya know.T)
Thats what i would do.