do you not just need a better crimp?
FWIW there are several things that may be helping the mudguard move
- not enough stays; you need a vertical stay on most rear mudguards; (a single set of stays somewhere near the back isn't good enough)
- wrong curve in the mudguard; i.e. the mudguard is strained somehow when it is fitted to the frame
- too much weight at the rear of the mudguard. IME anything more than a very lightweight reflector is liable to help the mudguard move.
if any/all of the above are bad enough, it won't matter how you fix the mudguard, it'll probably break.
FWIW an LBS mechanic once showed me a good dodge for better crimping (which will counteract moderate sliding tendencies); he took an old (and fairly blunt) set of large side cutters and nipped the folds in the bridge brace, in such a way as the freshly deformed region only just caught the edge of the mudguard. The resultant deformation not only grips the mudguard better, it also means that the ears on the brace resist being unfolded rather better than they would otherwise.
cheers
