Unless it is a minor mod, a carbon fuselage shouldn’t be cut/milled because you’re altering its mechanical properties.
It would be much safer to design an aluminium piece to insert between mast and fuselage. Usually, a new Gong mast is cheaper than this part … 
Are you sure the Nobile fuselage is made of carbon ? I thought Nobile only used aluminium in which case it could be milled like here:
You’ll have to create a new thread.