Do-It-Yourself, Repairs and Fixes

Hey, I just repaired one of my earbuds, which required me to open up the casing. Now the casing opens up way to easily at the seam, therefore I want to reseal it.

Can someone give me tips on what glue to use? I want the glue to be able to re-liquefy under a bit of heat treatment, as I may need to open it up again in the future, and it should be water-proof. The material is some kind of hard plastic.

Also some application tips and tricks would be nice, as the area is very very narrow and I don't have the steadiest of hands.