Well, it isn’t pretty, but after a whole lot of head-scratching and futzing, I now have the stepper motor attached to the turntable, and am able to rotate the turntable deck with minimal friction.
As you can see here, the biggest initial obstacle was the mass of wires and switch motors. To clear them, I had to extend the mounting beam a foot longer than expected. Once in place, I drilled the hole for the motor, then sanded it out a bit.
Yep, pretty much like that. At this stage I could tell there would be some friction issues to deal with between the deck and the well. Both components are plastic, and the slightest lean in the pivot mechanism sends one side of the deck screeching to a halt against the wall or floor of the well. I sanded some of the well, then I filed down the underside of the deck ends. That helped a little, but I could tell I’d need to place some shims under one end of the mounting beam.
Before I got into fine-tuning, I wanted to make sure everything was situated and solid. I attached the motor to the mounting beam with a couple of machine screws.
I then used CA glue to fix the iron axle inside the deck. Now the deck can be dropped in and pulled out at will by loosening or tightening two small inset hex bolts on the shaft coupler.
After putting a couple of shims between the baseboard and the mounting beam (to reduce a lean which caused one side of the deck to dip down while the other lifted), the deck could be moved slowly and without much friction.