You could screw or rivet a flat sheet of plastic (PVC, ABS or polystyrene) to both the body and the bumper.
For that matter, sheet metal with folded over edges would also work.
If you want to get fancy, a foam plug covered in fiberglass would be the slickest.
EDIT: There is this 911 project of interest (welded aluminum).
http://forums.pelicanparts.com/porsc...1s-1977-a.html