I've had success repairing my 95 W124 bumpers using scraps of stainless steel sheet glued in the back with Devcon Plastic Welder II. It is a 2 part structural acrylic that sticks well to a wide range of plastics. It is meant to be used with a special applicator but you can get away without it. Once mixed, get it applied fast as it starts to thicken very fast. Also check the best before date when you buy it as it doesn't have a long shelf life.
I lightly sandblasted the stainless to give a good surface to bite into.