As extra protection for my carbon frame, I bought this frame protector. Mounting it on the frame for the first time was a bit of a fiddle. I got some tips in the shop after which fitting it became a piece of cake.
This protector is easy to apply and offers your Thule clamp a lot of grip, much better and finer than the cloth I used to use for it. It also seems to distribute the pressure better. I can't verify that but have to take it for granted. Sounds logical. By the way, you never have to over-tighten the clamp anyway, a mistake many people make. Tightening it with care is sufficient. The most important thing is that the straps around the wheels are tight!