Summary

I wanted a generic hook to hold towels where the strap has been ripped off already or there is none.

When the towel has a corner that is a little thicker, it will not slip, I hope.
If the towel does not have a little "knob" at the corner, you can fold the edge in and tuck this in by folding again. (see photos)

The hook is meant to be used for rails up to 20mm. Otherwise just use FreeCAD and adjust the sketch parameters, it should scale accordingly