by Makulator, published
Problably not for general use, but may serve as a inspiration:
Curved slot nut fits into the slot of a canopy rail and keeps the curtain in place. Time from the start (wife complaining about direct sun in the evening) to completion <1h
Mirror part if needed, locking must occur against force of slider, of course.
The stopper locks itself because of the shape of the slot nut and the torsion applied.
Such plastic parts will not work with press fit only!
Spring tension because of press fit will vanish over time, plastic will creep when spring loaded constantly. Part shown needed almost brute force to insert for the first time. Now, after two weeks, it slides into position with almost no effort.