I find electrical tape to be very easy, very forgiving, and very clean looking to shim stuff with.
I tape multiple layers on the bottom of the thing that needs to be shimmed, then use and exacto knife to cut around the edges. If it's too thick, you remove a layer. If it's too thin, you add layers.
Sure you can argue with me about how plastic might not be good compared to wood, but I find the utility and efficiency of this to far outweigh any miniscule effect it'll have on tone of the guitar. Tone is in the fingers
Just thought I'd share a quick tip with you guys.