While installing new roller blinds, I broke the plastic clip that restrains the bottom of the chain loop (so children will have to work harder to hang themselves). This is a 3D printed substitute that seems to be even better than the original. The shape might be useful for any number of guides. It is designed in openscad, so its very easy to change dimensions for other purposes. The shape was inspired by roller chain links... easy to mount, strong and a good guiding surface.