(18MM coming soon!)
I needed an adapter one day and spent a little bit of time putting this together. I made it a little thick so you have a much easier to find thumb-area. If you don't want to use a choke, simply spin the dynavap away from the hole.
This is tight (but not too tight) on my Dynavap M. Depending on tolerances yours might be more or less tight. I printed about ten of these before settling into the tightness that worked for me.
If yours ends up too tight or loose, dial back your extrusion multiplier a little and it should be enough to offset it comfortably.
NO supports needed. The inside flanges inward via chamfers. Print it with the bong-socket-side down.