a bit of soil pipe with a mushroom cowl would probably do glued in the bitch mastic. you could the put a plug in the pipe to seal it.
Do you mean something like this?
http://www.toolstation.com/shop/p25397?table=no
And attach it to soil pipe and then use bitumastic?
I know. Just frustrated. But live and learn.TBF, probably for the same reason you won't.