Eric, I just used two pcs of Acrylic plate about 13mm thickness. The reason for such thickness is ridigity. I have used less thick stock....but have seen some flexing with thinner material....and clear acrylic allows for viewing of prism position. Simple !! I takes a steady hand to lower the top piece vertically....and to not disturbe the prism position....but B4 full clamp force is applied....reposition can be done !