Pete: You dont need to weld this repair. The patch that you will use is for asthetic purposes and does not require high mechanical strength.
This is a perfect application for the new industrial adhesives being used by body shops.
3M Body Panel Adhesive
But, I am assuming you can access the underside. If you can, clean and area around the hole, underneath. make sure its squeeky clean. Cut out a patch in some 20ga. mild steel and glue to the bottom to cover the hole and overlap. Clamp in place or rig some temporary braces and let dry according to mfg. instructions. When dry, clean off excess from the underside patch, prime and paint with your favorite rattle can colour. Now for the top, clean up any adhesive that oozed out the top, grind clean, fill with your typical catalized plastic filler, sand, prime and paint.