Sewing Machine Spool Pin Cap
If you've lost or broken the cap that keeps your thread spool on its rod and guides the thread out, this replaces it. Print one of the test sizes first to find what fits your machine's spool rod, then print the size matched to your spool. I've sized them for Gutermann 110m, 250m, and 1000m spools, though other brands close to those sizes should work too. It's on Printables if you want to make your own.