[morzh]

There are a couple of ways and then some.

My favourite two are:

1. If the cap has the cap (no pun intended) with a deep groove, I cut it around the groove, restuff and then glue the cap back.
2. In case the cap has no cap but has the groove at the bottom, and is held by a clamp, I cut above the groove so the cut would be hidden inside the clamp, then put a plastic pipe piece of reasonable length and fitting diameter inside and glue both halves to it.