Always wondered what I'd do if this dilemma presented itself. Guessing a shim would prove too crude an adjustment? Unless remarkably thin? . So, maybe? And, if the shim were to be that fine, also little to no risk of damaging anything, so maybe worth a go? Wonder if a thin paper one would do the job, on one side of the scope at the rear mount? Just thinking out aloud....... If not, maybe as Bellycrawler suggests......
Edit.....just thinking out aloud even more, maybe a cigarette paper shim?
Edit, edit......just trying to think of a shim material that would be ultra thin yet not likely to degrade........