Yup, I made a few hole punches by turning a chamfer on the end of a bolt and drilling the appropriate size hole in the end. I had a stock of gasket material of various thicknesses but I've run out of a sheet big enough to make the one that I want. SRM have replied saying that sometimes you have to trim them, but on the one I have, several of the holes are "hanging in the wind". More gasket material on the way!
One of the reasons I put the shim under the head of the bush is that I don't have suitable sized materials in stock to make a washer type shim, but was able to make one in two halves, which, as its encapsulated tightly between bush and inner cover, it won't be going anywhere. I'd have been content to do it your way, bb. Thanks for the input, which is much appreciated.👍