What is the Best Adhesive to use for this part of Powerbeats 3?

Any dual stage epoxy would do but it will be messy, smelly, hard to apply, and difficult to clean up…. but it will be air/water tight (assuming you applied it completely and correctly) and vibration proof (of this use case).

You can do it… don’t use a fast curing epoxy because you’ll want time to apply, attach, clean-up the epoxy. Make sure you have a decent tool like a toothpick that you can use to apply the epoxy to the edges easily.

Also, remove any and all other old adhesive that’s there so that the epoxy has the best chance to bond the plastics together.