I cut them out of wine corks so that it is only just possible to screw in the top and bottom. I also suggest you drop them in boiling water to soften them before you put them in and assemble them with a bit of soap as a lubricant. I don't cut the hole in them either preferring to clamp them all up tight in the open position then push through a red hot rod of an appropriate size.
this is obviously for the push slide round off hexag-on type of tap.
