Thumbrest designed to snap on to a tin whistle. You can customize it to the size of your whistle's barrel, and it works with tapered barrels.

The .stl file here fits my Clarke D whistles.

This is marked as a work in progress because I intend to go back and make it thinner and reshape the thumbrest a bit. It felt a bit bulky the first time I used it, but I quickly got used to it. I'm posting it as-is just in case I never get around to refining the design.

Yes

Thingiverse's customizer is more often broken than not. I recommend you download the .scad file and use the customizer built in to OpenSCAD instead. You can find several good text and video walkthroughs with a little googling. Here's one: https://www.dr-lex.be/3d-printing/customizer.html