How about this? I added these to my 3.0d a couple of years ago, dead simple. I welded up a small H-shaped carrier bracket which is pop-riveted onto the back of the box. The exhaust tip fits onto the bracket with a single long bolt, making it removable if neccesary for repairs. Very solid, no mods or welding to the box and the original exhaust vents to the bottom as normal. I can post pics of the deetails if anyone is interested.
 
 
 
 
Rob
2004 E53  3.0d