OK here it is-very rough. The material is 2mm thick alloy-the centre hole is 10mm diameter, and the inner 3 holes in a triangle match the backplate mounting screw location. The mount has been stamped or edm cut, judging by the edge marks. A thin slice 2-2.5mm thick from a piece of 35mm round bar would give you a suitable starting point. All holes are 2.8mm ID. These inner holes are counterbored 4.2mm (11/64" nominal) on the firewall face of the mount to provide clearance for the screw heads [this need not be done if you use countersunk screw heads...in which case obviously you countersink the mount to seat the screws flush. Obviously-depending on the thickness of your radial mounting flange you may have to use longer backplate securing screws....

ChrisM
'ffkiwi'