I have a bajillion rust fix pics scattered around if you need visual examples. Die grinder with cutoff wheel is good, I just do too much of this to not use a guarded tool. I think I used up 3 thick cutoff wheels and two grinder discs today. Anyway-
If you have a bench vise or some of those 3 or 4" wide-bill vise grip clamps, give it hell. I like to use the edge of the frame rack to hammer bends like that uniformly. Square pipe might help, to shape the metal around. Make the edge of your corner piece then butt weld that strip to the flat part from the back with the piece off the car. Nothing wrong with piecing together a patch like that.
On the bulk of it... hell some SUV roof skin or truck bedside metal might work for a longer piece. The stuff on the car is probably about 20 gauge, or around a mil thick. Don't bother with anything thicker, t'aint wuth it.
It looks like molding clips go on there. If thats mostly covered up by a molding (screw the clips on after), just make a row of plug welds to hold your new skin on. 5/16" holes at about 3" apart. Lay your patch over the repaired edges of the old and plug weld it on, Paint the inside of everything first. Seam sealer the outside of the seam then prime and paint exterior.
Of course the textbook way would be to butt weld the panel face seams but a row of plug welds smoothed with filler or sealer does the job neatly. Even if it doesn't look super-pro in the end, you have still added a LOT of value back to the car. If it went much longer it would get tougher to fix, thats for sure!
