Not much bonds well to plastic. Epoxy and fiberglass are possiblities. You may want to buy a block of plastic, hone it to the shape you want with some hand files, then fuse it to the old piece with some kind of plastic cement or solvent. It wouldn't be a bad idea to try backing the joint with some metal as well. You could even try making an entire, new mirror housing based on the old one out of fiberglass. I think hotrod.com or somebody did an article on making your own glass molds and parts to replace damaged headlight rings.