First step is building "main plate" - it is the part that holds ball bearing, motor, and some other parts. This part can be cut from some flat plastic plate. I have used 5mm acrylic glass:

Drilling 42 mm hole for bearing was a little tricky.
This part can be printed at 3D printer.

Next step is assembling everything.

Photos of assembled mechanics (top view):

Side view:

Motor is connected with the PCB holder with rubber belt.

Down view:

Here it can be seen method of fixing the PCB holder to the ball  bearing.

Next step is to fix encoder at the "main plate" and to test it.