Hi Guys,
Not sure if this will help at all but I had some custom work done to hold subs/amps/crossovers/etc in our first X5 back in like 2001. Granted this was when I was much more into car audio so our new 2006 is pretty plain back in the cargo area. Less the spare (come on we have roadside!!!), my old setup didn;t take up ANY cargo space yet provided ample volume for two 12'' subs and electronics. It wasn't a very "high-end" system but it was good gear properly installed and tuned and sounded great.
Good luck with yours!
PS - My plan for our new 2006 3.0 is too simply add a single 10 or 12 in a small sealed enclosure that is easily removed from the X when we need to pack her up tight for tailgating or travel. Pull two speaker wires off and the sub box comes right out!