Recently Polyvance(formerly Urethane Supply Co.) showed off some new techniques for repairing plastic bumper reinforcements. The bumper they repair in the video is from a Mazda Miata, but the technique could save any bumper cover that uses a similar method of reinforcement.
Hopefully you can use this information to salvage what could be a lost bumper cover. This method can be used on any cover that uses reinforcements that are made of the same material as the rest of the cover.
Let us know if you want to see what a Polyvance Welder can do for you.