I made an L profile of some steel, drilled two holes. Screwed it to the frame and to the fender (drilled a hole in the fender too). Placed it where the rack would be mounted - the angled part of the rear triangle. So the rear fender is held at 3 places now: near BB, in the middle (up) and all the way in the rear.

I can post pics. You need some basic tools, or a mate that has them. Way to get an L shaped piece of steel/iron, and a drill for metal.