I think going for the bendy willow approach will be better than the make rigid approach, so it can snap again.
I suggest using just two fixing points
1. The rear one
2. The front one under the seat
You may use the plastic sleeve onto the metal plate as well, if the plastic has not already failed
Use an oversize washer to spread the load around the plastic at the front (or replace the missing plastic). You should find that you can get enough clamping force to keep the cover in place, but that it will deflect enough when shuffled over