What I use is a product called Kwik-Patch. It is meant to be used to fill in small nicks & scratches in a ball's surface. It is a very thick gel, very similar to super glue. It comes with an accelerator(to speed up the cure time), and it sets up VERY hard in about 5 minutes. Since I started using it to glue in slugs, I've had only 1 come loose, and that was about 2 years after installation.
drillbit