I just started on this over the weekend. Right now, I'm just doing the mock up using an old MDF shelf; getting the necessary measurements and working on the layout for the electronics. (The final version of the plinth will of course be much more substantial.) As seen here, the electronics can be completely separated from the motor unit. This should be fairly straight forward.
A 2" diameter hole easily accommodates the bearing.
The flange screws will soon be replaced with bolts and nuts. The original arm will not be carried over, as it is long gone. A larger plinth will also open up 12" arm possibilities.
Soon a 1" diameter hole will be add to enable the control wires to feed up to the motor connection.
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