WORK IN PROGRESS!!!! USE AT YOUR OWN RISK!!!!!
I really hate paying people to work on my car, and cars sometimes need new tires. If you mount tires yourself, this should allow you to complete the job and balance them as well.
This flat side should (have not measured yet) mount to the inner side of your wheel, conical part should terminate near the center of gravity. The center hole is sized to accept 608zz (22mm OD, 8mm ID) bearings with an 8mm steel rod through the middle. Use at least 2 bearings, one on either end of the central hole.
Mounted up, this should allow the wheel to spin freely and allow the detection of any heavy spots to balance out.
I HAVE NOT TESTED THIS DESIGN YET.
I will update with test results whenever I get around to it. If you decide to make one, please double check the sizing prior to printing and let me know how it turned out!!!
Made holes a bit bigger and reuploaded