Nice angle plates and thanks for sharing, I made something similar a few years ago but borrowed my friends angleplate when machining the two large sides.
Norman, when I did this many years ago I started by machining a pair of circular squares on the lathe, clamped them vertically to the mill table, and then clamped the square to be machined to those. That way you don't need an accurate square to start with.
Oh, them. Then you don't indeed need an accurate square to start with, but you do need a pair of accurately 'right' cylinders. That's right as in right triangle.