A simple sculpting wheel designed for two ball bearings (radius ~5.9/18.6mm). You may need to adjust the ball bearing size to fit what you can get hold of - source included.

Ultimaker

Ultimaker 2

No

No

0.1mm

44%

The important thing is strength, and decent fit for the bearings. Mine broke and had to be glued because I used too little infill, hence the larger number cited.