It is just a flat 15mm deep spacer with the two holes drilled. In essence half an annulus with a flattened side.
In case you can't read the figures I make the inner radius 142mm, outer 172mm, the holes are on a 149.5mm radius.
Each hole is 12mm diameter, and the top hole is 54mm from the flat side, bottom is 45mm from the flat side, and the flattened side shrinks to 10mm at the thinnest point.
I've had a quote that works out at about £30 each ex works if I make a small batch of 4 or 5 - which is possibly worth the effort.
If a small number of people are interested in one I could get these made.
Or better still does anyone have any access to laser or plasma cutting and want to manufacture them?
If we were to go forward it would be worth someone checking my drawing or making suggestions to improve it.