thingiverse
I didn't see any classic CTR ring designs on here so I thought I would give it a try with my own design. Downloaded an SVG file of the CTR symbol and kept the model pretty simple.
The Size 6 ring uploaded has a .65" ID
Direct link to the original creator's page
Click the "View on thingiverse" button above to visit the original model page on thingiverse. You can download the STL file directly from the creator's page for free.
This STL file is compatible with most FDM 3D printers (Creality Ender 3, Prusa MK3S+, Bambu Lab, etc.) and resin printers (Elegoo, Anycubic). Check the original page for recommended print settings and materials.
Yes, this model is available as a free download on thingiverse. Some creators accept tips or donations.
Most STL files can be modified using free software like Blender, TinkerCAD, or Meshmixer. Check the license on the original thingiverse page to see if modifications are permitted by the creator.