Okay, so I sat down this evening and drew up another, smaller idea.  This one has an 8" disc and is about 24" tall to the top of the shaft.  I got permission today to pick through some of the material at work, and I'll be ordering some laser cut plates. 

This design has a counterweight that balances the rotating assembly, so it will be harder to get it moving because of the extra mass but it should stay rotating longer because the mass is concentric to the axis.  With the crosshairs cutouts in the target plate, it should help reduce air drag a little.  This whole design is steel, with a three-pronged spiked stand that is driven into the ground to anchor it.

Cool idea.  Friction is always going to be a challenge on a rig like this, but if you design in bearings and a separate braking mechanism you’ll get more consistent results… that may be splitting hairs too early though.  The best thing to do is build it, try it, learn from it, and then build it again based on lessons learned.