You can always try the Capella plans from the link below. I built it and it works fairly well. I find the friction required to get a proper static/stable lock on anything insufficient. Could be my implementation of the plans. I'm modifying the concept to use a couple of worm gears to stabilize everything. I've purchased some worm gears for garage door openers with 0.5" shafts. They are sturdy enough to keep a lock on an object. Probably not accurate enough to track an object over an extended period of time but, this is just for fun right?
have a look see: http://solar.physics.montana.edu/larson/Capella/index.html