Clipper bumper repair.

Evening guys. After a bit of advice! I have a 1990 gti convertible what I'm using for the summer whilst doing bits of body work. The issue I have is on the bumpers where the wheel arches pop rivet onto bumper the lip has come adrift from the bumper! I have tried fibreglass but was not successful!! Has anyone else had the Sami problem and how did you get over it? Thank you. 

is the wheel arch hole larger than the rivet?
I think I would try using a larger metal piece or plate a Fender washer as a backing plate and then trying to get the rivet to hold.

If it is because the Arch hole is larger than going to an oversize rivet, and or a small bolt and nylon locking bolt may work.  Hard to visualize with out pictures of your attempted repair.