There are many solutions to adapt the optics to a diagonal, but the adjustment system of the entire block is more problematic. Here is a simple solution using 2 metal brackets to form a stirrup.
The diagonal is taken in pincers and immobilized in the plier, by moderately tightening the 3 bolts.
Some holes have been enlarged to allow the insertion of the 3 bolts
(6.5mm holes), as well as the 4 adjustment rods (8mm holes) giving
sufficient movement to allow fine and progressive adjustment.