I've had really good luck with epoxy, but make shure its the stuff called Plastic weld. It seems to bond much better than standard 2 part epoxy. I've used this on all my fiberglass armor, and never had a problem. It comes in a dual tube applicator. You just squeeze it out & mix it up. You can get it at any walmart. Hope that helps
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EDIT: BTW, if you are making your knee darts out of spacers, I would glue them together 1st, then drill a hole through the center of the entire knee dart and put a pin, (cutt off bolt or whatever) through the dart w/ epoxy as well. That way you know for shure they aren't coming apart