I'm interested in this too. I have the same issue on both sides. I thought about cutting out the metal and welding a flat piece in instead of matching the original contour. It will get covered up again once I figure out a bumper anyway...
Temp solution could be some duct tape and cardboard. Or cardboard and fiberglass?
I just found the exact same thing a month ago but on pass side. I'm going to remove the bumper, grind rust, weld sheet metal from Lowes and treat it heavily so it doesn't happen again, the other side is getting protected too!
only proper way to fix it is to weld in new sheet metal flush. Most likely the rust goes further than it shows as the metal will thin some as it rusts outward and all that thin metal needs to be cut out to weld to good solid metal. A good temp fix would be to wire brush it well, use phosphoric acid etch on it to kill the rust, tri sodium phosphate to clean and then a thick coat of paint. Eastwood makes a heavy duty anti rust paint that is a great final touch once you get the rear end all sorted out. It is like a waxy oil paint that wicks into all the crevaces and prevents future rust. I use it religiously on all my rear sill repairs.
I have not used many of them, I don't do bondo work or paint, don't like it, just metal work, but that eastwood heavy duty anti rust is some cool stuff. It is best to think of it like a top coat once an area is painted. Great for spraying inside sills and in areas where metal overlaps itself as it will wick into tight places and create a waterproof oily/waxy barrier.
In these areas I typically patch with stainless steel. You really cannot get inside and treat the inside of the panal, so the moment you fix it, rust will restart. using stainless rod, I have opened up sections and they did not rust after a year at the joint.
best about this area, is that you never see it with a bumper covering it.
Post up on the TAC board, hit clubhouses, central, Tornado alley cruisers and see if anyone will help you out.