The shim is located in a pocket created by drilling a hole
in a 1.2mm Brass sheet. This sheet is then glued to a 3mm Brass sheet with,
you've got it, Chemical Metal. The sandwich was then drilled to take the four
fixing screws and cut to the correct shape. I made a gasket from 0.4mm gasket
paper and located the shim in the pocket with a spot of silicone instant
gasket. If you don’t do this it tends to stick to the spindle when lifted, due
to oil viscosity and ends up out of its location. You would have to do a strip
down to relocate it every time so it’s worth making sure it stays put.