Step 10: Clamp on the roundess indicator.
I originally thought to simply super-glue this on, too, but I realized that it wouldn't be adjustable for different rim widths. Thus the clamp, which allows you to move the indicator to suit any wheel. And believe me, this little clamp is plenty strong enough to hold the bolt solidly.
Note that you can adjust this setup to any angle. It doesn't have to point straight up and down, as long as the tip of the indicator can touch edge of the rim.