More detailed design drawing is attached.
I decided to machine new nut for counterweight shaft, and use modified ADM dovetail bar (leftover, my C11 came with two of them), fixed to the RA axle. Also I intend to use surplus EQ6 saddle (I am using ADM-style saddle for C11) for Canon 400mm and Bintel vixen dovetail adapter for lighter Rubinar 10/1000 (or whatever).
I hope for the best
Cheaper alternative is to use
2 aluminium 10 or 12 mm square bars instead of Bintel clamp and fix them with screws on the ADM plate.
To achieve scope parallelism, only one dovetail clamp needs to be rotatable around mounting screw.