Depending on how much of it you need, maybe you can find a contour that would work in the back using those?
I don't think the front patches would be much use on the rear. The rear of the fender is a compound curve (dished) in most areas that rust out, and the front patch is basically a flat curve (single direction) with a flange. The main part of the rear rust-out isn't hard to form with a hammer and sandbag, but some shrinking is needed to get the compound curve formed. I was able to form patches for the rear of both my rear fenders using stump shrinking on my first attempt.
