I like my Juki sewing machine (an old TL-98E). The spool cap was getting too loose so I designed and made another. I used Blender to design it. Blender has a lot of power that makes it good for 3D development. I made it in PLA but PETG would probably work better.

I printed this in PLA, it worked fine. PETG might work better.
it is designed for a 6mm diameter thread spool holder, scaling diameter up or down would make it fit other sizes. Scaling height could make it work for other machines, not all sewing machines have the same height or diameter spool holder rods.