I printed this in Nylon (bad finish because of 0 cooling but strong as heck) buy I assume this would also work in ABS or hard TPU. PLA I'm not sure because you need some flex for it to be able to clamp nicely

Print with supports in this orientation for highest strength

The most important dimensions you can find in the pictures. Also included the Fusion 360 file if you'd want to modify for your curtain rail