The diameter is 78mm, or 3 1/16".
The bevel starts at 70mm or 2 3/4".
The thickness is 6mm or 1/4".
The bevel stops at 2mm or a hint over 1/16".

This is measured off the Lapla 50, wihch holds my Rodagon 180mm to my Tripla triple turret.

If you're having one made for a 210mm Rodagon, I'd be very interested in getting one of these for myself... As well as one with a 29mm thread for the 60mm Componon