Greetings to you and Borris!
A carbon fiber shell would be cool! In order to answer your question, I have thought for a few days, but I found it’s difficult. The interference with WiFi is very tricky, depends on the specific material and shape of the obstruction. It cannot be generally answered without tested.
I made a chrome-plated shell of HEXA. I didn’t find that its WiFi signal was affected. It works normally. I hope this information helps, since the plated shell is also a conductor like carbon fiber. I really want to know if carbon fiber can work too. I hope you can try it and tell me.