This is a good toy to demonstrate centripetal acceleration. The torus also makes the marble less likely to become a projectile, compared to a banked track. This could also be used for conservation of energy, as an analog to orbits, because friction and wind resistance are small with the marble.

I printed mine with 2 perimeters and 10% infill, with 3 top and 3 bottom layers. I had to scale to 93% to fit on the print bed.