<<If it's got a crack in it, but otherwise OK, take it into your local
hardware store and find a PVC end cap that the water tank cap fits into,
and glue it inside that. bob>>

Why all the messing around?

Pick up the phone, call VW parts and order a new cap.

No gluing, no schlupping around in a hardware store hoping you'll find
the right piece to make a short story long.

Way too many moves to make a super simple job way too involved.

And the best part is; it won't be right when your all done anyway.

Crazy.