IKEA Curtain Hook
It's a hook for IKEA curtains, but I believe it will fit other curtains as well since it's a standard size. If needed, you can adjust it to the length in one dimension.
I recommend printing the hook from PLA or another strong material with 100% infill or with the strength setting on the Bambu Lab slicer.
The Story Behind: We bought curtains from IKEA and wanted to install them on a ceiling-mounted track system, but we found out we didn’t have the proper accessories :D And of course, it was late in the evening and the curtains had to be installed right away! :) So, I designed and tested a single hook, it fit, and then we went ahead and installed the rest. Now the curtains hold their shape nicely.

Design by Robiovic on MakerWorld (license: BY).