After drilling the platter for a Noval tube, I finally made a fixture to hold the platters firmly, but gently, in the proper position for drilling:
The platter sits more-or-less flush with the surface, where credit-card plastic pads work fine. Thinner platters may require compliant padding.
The solid model has locating pips at ±50 mm from the center and airspace below the platter for the drill bit:
The 1.16 inch hole spacing matches the Sherline’s tooling plate. The center hole seemed like a Good Idea, although it has no purpose right now.
The OpenSCAD source code is the same as before; just set
Layout = PlatterFixture; and it’ll produce the right thing.