Foxy

Sure, it's not as accurate as a balancing rig, but some thing like this would do the trick (see 2nd pic), and is simple to construct. You mount the wheel in the middle, and the heaviest part will always fall to the bottom. To make it even better, you could weld something up like the first pic and put the wheel (or both ends of the bar, whatever's easier) on a high quality ball bearing. When the wheel settles at the bottom, add a touch of weight to the upper most inner rim.