Mark,
Nut and lockplate detail, as requested ……….
Modified nut :

And this is my solution for the locking plate (made from a flattened flywheel locking plate). Note it locks the six damper fixings as well as the main nut :


N.B. – To ensure the slots align correctly with 3 of the flats on the nut, they can only be cut once the central nut is fully tightened. To determine the correct slot positions, temporarily attach the plate by two or three of the six fixings then the edges of the nut can be located and marked by gently tapping the plate with a ball peen hammer.
B…………