are you talking about rear plates or front or both?I've been trying to find a product online that is geared towards protecting your paint's finish from the license plate's potentially damaging edges.
After looking with no success, I realized that it likely wasn't that simple... In addition to insulating the paint against friction and movement by the plate, the material must not retain moisture.
Note: As I do not have a front plate installed, this only pertains to the rear at the moment.
At this point, I'm thinking a decent clear cover to keep the plate clean, and isolation washers for the back side.
What options have you all tried?