If the shell does come off the extrusion with the extrusion still bolted to the car then that makes it much more feasible. Still a chunk of work, but possible.
A8 parts have a black one listed at the moment (at half price), still attached to the extrusion. May be worth buying that to have a good look how it comes apart and decide from there?
http://www.ebay.co.uk/itm/like/Audi-...d/130742862326