This is a tiny plastic thing that can be pressed into a curtain rail and will stopp the curtain gliders at a specified point. It is designed for a curtain rail with a 6mm rail gap, but can be scaled to pretty much any rail. At 100% scale it will be quite snug in a 6mm rail.
Print several at the same time to allow the layers to cool between z steps. Worked fine with a 0.4mm nozzle, 0.1 layer height in ASA material.