This script generates comfort-fit rings in a wide variety of sizes. I wrote it to make customized ring shanks that I can combine with other 3D printed pieces or carved wax to use in lost wax casting. That is how I made the sterling silver ring in the first photo.
To make your own ring, click "Open in Customizer". The optional flat top makes it easy to attach things to the ring. If you don't want the flat top, set Top width = 0 in the Customizer.
This is the North American version. It allows you to specify the ring sizes used in the US, Canada, and Mexico, from size 0 to size 20 in quarter size increments. There is also an ISO metric version here: https://www.thingiverse.com/thing:2936960
The ISO version allows you to specify the ring sizes used in most European countries. This page has a ring size conversion chart for other countries: https://en.wikipedia.org/wiki/Ring_size
The wider the band, the tighter the ring feels. Comfort fit rings feel like a narrow band, so your comfort fit ring size might be a quarter size or half size smaller than your conventional ring size.