Things I did:

1. I first tried prototyping using clear plastic transparency sheets looped around the aluminum tube, but that had too much friction and also was dangerously close to peeling off the wall.
2. Next, after finding that string works well, I prototyped using cardboard, aluminum flashing, string, tape, etc.
3. Finally, after having a good idea of the design I wanted, I CADed / 3D printed the parts.