3D Printing can help us build things easier. Here’s a jig for mounting the tertiary mirror:
The mirror sits flat, face down. Small slots hold 2mm shafts for spacing. The slots create a range of heights for the shafts so we don’t have to worry about the exact thickness of the mirror or creating a hole that fits the shaft perfectly–gravity takes care of the fine adjustment. The mount is now keyed for orientation and you can see a little notch in the back of the jig where this happens.