car bumpers are made from various different plastics ie pp ppu pl the list goes on to about 8, it will tell you on the back of the bumper imprinted, so idealy you want the same plastic, then you can cut and splice them together wth a plastic welder, there are some cheapo kits on ebay, they are like a heatgun with a feeder tube with sticks of diff plastic rods, or use a soldering iron that ive used in the past, trim bits of exceess material that is not used for the rods from behind the bumper, and fuse it together, then to reinforce i use a good 2k adhesve on top of the welds, dont use fibreglass it wont stick and will crack and fall off, Ant!!