I know cost is always a consideration, and touch sensors are another way to go, but with regards to a more traditional setup block, my first article samples from my hobby project to test out on-demand manufacturing just arrived and they look pretty good. I posted a photo over here: CNC Milling a ShopBot XYZ Finder via Xometry Service - Review and will update with more details in the next day or so.
I think some CNC milled calibration blocks or tools might be closer to being viable than I certainly would have thought a few months ago. I know having a known perfect reference of some type is a challenge we’ve kicked around lots…
-Jeff