I used to have a Wheel Massage Device with four wheels and using Freecad I designed a digital 3D print version. It's 12 cm long so it fits most hands. After assembly with some glue it's ready to use.
20 % or higher for strength
The wheels are a little flat on the side so they will stick to the plate, and after printing you can put some glue in the holes of the balls and assemble the device.