The fiberglass "bumper" is nothing more than a skin, and will need to be attached to a reinforcement beam if you want it to provide any protection.
Here's a pic with some metal strips fiberglassed into the back of the bumper. I believe that it came form one our forum members, but I don't remember who.
You can see that the metal strips have bolts attached so it can be fastened to a reinforcement beam of some sort.