Stray Rust Strut
It seems to me that the rear struts on e21’s have a tendency to rot more than other chassis. If you're in the market for one of these, be sure to give attention to this area.
The rear firewall needs some cutting away just to get proper access to make the repairs, here. Be sure to cut straight and clean to make life easier later on, when you replace those spots. Don't lose them!
Working with flat stock to make strange shapes, often times you can do what needs with small relief cuts, and then massaging your radius with a hammer. In an area like this where the seats will be hiding the patch, those cuts and bends will work a treat. They aren't pretty enough to use anywhere you'll see, but they are plenty good to do this job, and fast enough to keep our time and money budget in check.
The flap grinder will take that pride out.
Poor man's CAD (Cardboard assisted drawing).
Patch, liberated from the sheet.
Tack one edge.
Relief cuts, beat, tack as you go.
Soon, you'll have a solid strut tower!