@bazbt3@cgiffard It turns out that a cradle that is 100 degrees is just about ideal for minimizing glare from an overhead light reflected onto cameras at the sides. So each side needs to be 50 degrees to make that angle.
@cgiffard All of these wet glass grinders use diamond grinding wheels. Oddly, I read that it feels smooth to the touch and isn't nearly as dangerous as woodworking sanders and grinders that can take a hand off.
@cgiffard Here is a picture of the finished miter. It looks pretty perfect. I think the line is just ever so slightly off from parallel with the opposing edge. But for the book scanner, it is close enough.