I know EXACTLY what you are going though. In the end I did the following:

I measured the taper, scribed it onto a pattern, then machined a picece of brass to fit as a shim.
The brass is soft enough that it doesn't scratch (it will discolor under the mount but if you ever take the mount off solvent cleans that right up). Also the brass seemed to be less temperature sensitive than the epoxy stuff I tried...on hot days it would get a bit gummy and on colder days it would really harden up.