I started with a quick sketch to get components and boreholes placed:

Next I cut out two discs dia. 80 mm from a 3 mm aluminum sheet with a coping saw. A metal lathe or a laser cutter would have been nice, but I don't own one of these machines, so a lot of grinding and sanding was required. The trick is to fix the discs with a screw and a nut on the chuck of a drill press and use a file and sanding paper to smooth the edges while they are rotating.

Further progress:

Base rotate unit successfully assembled on upper body: