Just for fun, this is the design - workpiece is 8x8x0.5" HDPE, three passes, 3d pocket to rough out the discs (and center hole), 3d contour to make sure the verticals are “true and square” (not), and then a final pocket operation to cut an axial-face groove concentric to the center hole and outer diameter.
This is a transmission differential setup tool (NLA from mfr) where I need to project the ring-gear pitch diameter into free-space along an axis through the differential bearing carrier bore holes in the side case…to check the pinion depth per the mfg spec.
The small disc drops into the bearing carrier bore in the side of the case, and the large disc is a reference diameter which then sits in free-space in front of the pinion head. A piece of 2" OD steel tubing is cut to length and faced on both ends and fits into the axial slots/bores to extend the axis through the case and position the large disc in place.