I simply used an 8 foot closet pole, the kind normally used in a closet to hang hangers on.  It's a good diameter at 1.25 inch, relatively lightweight and quite stiff as it's thin steel.  They do make a heavy duty version that is much heavier, but I'm not putting that much load on it, despite the long span.

I also considered PVC piping or electrical conduit.  The PVC, while easier to drill, needed to be much larger diameter in order to not flex much, which would increase the motor torque and require a slower motor.  The electrical conduit was much heavier.

I also picked up a pair of end caps for the pole.  They are surprisingly thick plastic and fit on quite firmly.  I intended to use one to mount a bolt in to drive the pole, but ended up using the other cut down as a shaft collect to keep the pole from sliding end to end.  I think they are PTFE, which is super easy to work with.

To couple to the motor, I tapped a 1/4-20 thread in the middle of an end cap, and bolted on a bolt with locking nuts on each side and a lock washer.  The bolt head sticks out about 3/4 inch, with the nut on that side lined up. This allows a long socket bit to be mounted on the drill, and yet the pole can be easily slid out or installed.  The hex bolt and socket form a nice coupling with easy moment, yet plenty of torque.

For the collet, I cut the cap off and threaded a locking screw in the side, though it was probably tight enough to not need it.