We found an awesome lazy susan bearing at the local Reno Depot. It had a 12.5" outer diameter, and viewing from the top it had a 3/4"-wide fixed ring, encircled by a slightly higher 3/4"-wide rotating ring.
Our turret was all mounted on an extrusion frame, so we bolted that to the rotating ring. A Globe motor mounted on the turret drove a rubber cylinder against some belt we glued onto the fixed ring, for turret rotation by friction drive.
All the wires ran up through the middle of the bearing (the balls went in the front).
Here's a picture of the robot, if that helps make things any clearer.