That whole section is rotten. Easiest thing to do would be to see if there is a partial replacement panel available for whatever that vehicle is. (Porsche 944???)
can pretty much guarantee the back of that panel is much worse than the front, probably at leas 6 inches.
Metal replacement and add a damn drain hole in that corner.
Best bet is to go to one of the many Porsche Forums and ask around. Doesn't look like they are being produced in any real quantity though. There are small companies and individuals reproducing some body parts for them. Finding out who that is, will be easier on a 944 forum.
Buying a used panel is an option as well. Car-part.com is used by every salvage yard in North America (nearly) https://www.car-part.com/
Before you buy anything though do a very thourgough inspection of the car for any additional rust. Unibody rails, floor pans, around the lips of the 1/4 panels, rocker panels. Make sure you have a pretty good idea of the extent of the rust before you buy anything.