I don't see any need for metal under the antenna. I had my XM antenna mounted on the rear shelf of my Passat for about 8 months... just loose and sliding around on the carpet. I had to move it back in position every so often, depending on how spirited my driving was. My reception was generally very good. Only when driving on roads with mountains right in the way of the sat signal did I have real problems... and that would probably have happened on the roof as well.
Now that I have my 330i, I mounted the antenna in place of the center tether anchor. Again, my reception was very good... even though it was "pulled back" so far from the glass.
The only problems I'm experiencing is now that I've tinted my windows. I had a Solar Guard True Gray tint put in place which I believe may have some metallic properties, because my signal will drop more often, especially when traveling east or west. It can be somewhat annoying. I didn't have this problem with my Passat and the Madico tint I used there, so I may have the back window re-tinted with a non-metallic film to correct the problem.
Eric (the Obsessed)
2002 M3 Convertible
2005 VW Passat 1.8T Wagon