Update: I recommend version 2 at https://www.thingiverse.com/thing:3713421
Simple configurable vac adapter. Specify the diameter for both adapter ends, the end type (male or female), amount of taper (or none), and the wall thickness.
update 6/10/19: Bug fix. Also added a slight taper on either end so the adapter can be pressed on tight.
update 6/10/19: made taper amount adjustable on either end.
update 6/20/19: added length adjuster