Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:887715.

A manual marbletrack. Just place a standard glass marble (mine are 16.5 mm diameter) and it will roll to the bottom. I have to fight the urge to keep doing it... https://www.youtube.com/watch?v=BSx1ZfvO8sc

** Now (July 2022) with the OpenSCAD code for personal customization. ** (NB: best to download and use locally, the Thingiverse Customizer doesn't do it)

I wanted to stress my printer, and a spiral came to mind. After printing a flat spiral I thought of using the "track" for something. Thing #58406 was the baseline for the second code version.