Mount the plywood disk on the upright using one of the middle holes as a pivot point, and mark the upright through the middle of the curved slot. If you measured the placement of the two pivot points on the routing jig (which I didn't :-( ) you should end up with the mark roughly between the holes in the upright that act as pivot points.
Drill a hole on the mark that is somewhat smaller than the shaft of the wingbolt that will be used to prevent the disk from rotating, and use a tap to create threads in the upright. Having the ability to hold the disk in place means that you can be approximate with setting the position of the counterweight, and you don't have to fiddle with it to make it perfect. You can repeat the process for as many pivot holes as you want, although in truth, you probably won't need more than one or two.
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