I've done this...I have a smoothed down grille which has been wrapped in CF. I am using a numberplate plinth, but it has magnets on it so it is held magnetically to the grille. The holes in the plinth were a good size (5p piece) for magnets and there's no worry about them detaching themselves as they sit in the recesses.
The. It's just gluing magnets to the back of the grille and that's it. Some really strong magnets are needed, but after 1k miles, the plinth is still there.
