I would just remove what is rotten and either fabricate a piece to patch or like said cut up a pattern panel to fit hole, either butt weld or if it will be hidden use a joggler on one panel to give a flush repair from one side, just make sure you seal and paint both sides to make it last. I did my Fiesta in this way, even just cutting out small sections of inner and outer wheelarch and just fabricating inner panel and cut up a repair wheel arch panel to repair outer. but when I noticed the rear panel had corroded a bit I just bought a valence and cut off the old completely and put on a new panel, depends what is easier when you look at rot, as some panels are easier to replace than repair, but always cut out all rot and seal new metal properly for a long lasting repair. Weld through primer can help if you cannot get to back of panel after its been welded on. Just waxoyl everything as well.

hope this helps