I've never glued one in a rubber ball, but I think it would be the same as in a resin ball.  First, drill the thumb hole shallow enough that the slug bottoms out with at least 1/4" sticking out of the hole.  Use ~100 grit sandpaper to rough up the outside of the slug and the inside of the hole.  Hammer the slug into the hole about 3/4 of the way, then spread super glue around the top inch or so of the slug (don't get the glue in the vent slit).  Then hammer the slug the rest of the way in until it bottoms out and let the glue set up before drilling.  --  JohnP