Morelock

That's it. There are a couple of different size carbon rails fyi, I know some of the Fizik's are bigger/smaller than most.

Strange to have seen it slipping with metal rails as well. This is the same topper a lot of TT/tri (P5,etc) bikes switched to last generation and they generally get glowing reviews. I've wrenched a couple of them and never had anyone complain about slipping. Is there any chance the bolt is loosening? Checked the threads/use blue loctite?

I still think the right piece for the carbon rails would help, but I also think you should disassemble everything on the topper and look for anything that seems out of place. The metal rails should have been rock solid.