Pinball plastics take damage from the mounting nuts; they get scratched, and can deform.
This clear polycarbonate washer can be placed between a plastic and its locknut to protect the plastic surface and even out the clamping force. Use with plastic protectors, or a nylon washer, underneath. Mosts post are #6 size, a few are #8 - so we are stocking these. I have to apologize for the high price, but these things are expensive! Fortunately, you only need a few of them.