Thanks for sharing your method! I don't have collets, so I may just try and make my own bushings out of brass or HDPE.
Have you tried a stem with stabilized wood? I'm not sure how it would do with the heat, but using a brass tube inside the stem where the tip connects might be enough to keep it...
I've been wanting to make some of my own stems on the lathe. Do people mostly use pen mandrels to turn stems on the lathe? What bushings do they use? I've had a hard time finding 8mm ID bushings that fit on a 7mm mandrel.
Thanks for the replies! Looks like my Omnivap has a bent condenser tube so it never fully closes. I've already sent an email since it arrived like this. Thanks to your great explanations I still have got it to work better than it was.