Some of those bumpers are made out of PVC, ABS and some are TPO (Thermal Plastic Olefin). The TPO ones are not compatible with fiberglass. The glass mat and resin will just pop off after short while. You need to have the piece 'plastic welded' and filled with a filler made for the TPO, with lots of adhesion promoter. Before you start on the repair, you might want to find out what your bumper is actually made of.
Roush has their rear valances on sale right now-
Mustang GT Rear Valance for RH (1999-2004)