Ya, the whole problem with bondo on fiberglass is bondo doesnt flex, fiberglass does. Itll work, and you can make it look good, but if you were to just barely bump into somthin with your bumper thatls most likely cause it to crack.
Try some aluminum flat bar. Light, cheap, and easy to form/drill without special tools. you can get some 1/8"x1" for dirt cheap. And you can buy it by the inch so you dont hafta buy a bunch that you dont need.