Understood, ic-racer. But I have a couple problems with that. First, I have to get in my truck with tripod, camera, and tools and drive a long way before there is any horizon. Further, I've been doing these sorts of home-baked ideas for too long, and am moving on in my repair studies to wanting to devise bench-testing variations of expensive and unobtainable factory fixtures and jigs. And do better work than these slow cumbersome and imprecise tripod tests on a pretty day.
I wish I could better understand the construction of the official Hasselblad mirror jig, but have never seen one. But I can surmise how it must work and come up with a variation. Reverse engineering, I suppose.