6" diameter flat sanding disk/lap with 1"-14 threads (might need a tap or thread chaser to clean up the threads depending on how your slicer deals with the geometry, but its almost good enough as is)

I'm using harbor freight spray PSA glue and wet/dry sandpaper on my cabochon machine.

It needs some work as the edge of the plate got the shakes pretty quickly so a revision is coming up.