After installing a new shower rail, I discovered that the fitting for the shower head was too small. Cue building a parametric model for an adaptor.

This version of the model works for a female connector with a diameter of 19mm, and a shower-hear male connector tapered from 21mm (at the bottom) to 24mm and a hose diameter of ~15mm.

I'd recommend using PETG due to the temperature and dampness encountered in a shower. I have not tried other filament types.