Inspired by @flackenstein's plug.
I wanted a single piece plug. And I wanted step files instead of STL. So I got the calipers out and designed one from scratch in Fusion 360.
The Fusion 360 design is parametric, have fun changing variables.
UPDATE: A new version of this model with twist open-close functionality is available - https://www.printables.com/model/549696-eibos-cyclopes-twist-open-close-vent-plug
The author marked this model as their own original creation.