Sometimes, you might need to shim an interface somewhere, like under a footer or cone shelf protector, between a shelf and shelf support, or under a fifth Fat Dot.
I've shimmed and leveled many times using Post-It Pad paper and never detected any sonic detriment. Just tear off as many sheets as you need (maybe add one more for extra measure), trim for neat appearance, and you're in business.
Regards,
Steve Herbelin
Herbie's Audio Lab