Thanks for the nice words
The first prism rotates roughly around its center. It sits on a .25 brass post. I wanted to get the second prism the same way. Unfortunately I didn't have enough room. So, the second prism sits on a small table attached to the post. Before I drilled the holes for the posts I did a powered mock-up get a rough idea where they should go. Lot easier than doing the geometry

I knew the first one should go on center with the beam but I really needed to experiment for the second one. Your idea sound really cool. Please post some pictures when its finished.