This info and photo is from Lemans. Sorry to use your photo without permission Lemans but I really like the looks of that cargo box (and your X5 of course). I'm thinking of getting the same thing when I get my E70. Like you Buschy, I also am getting the 3rd row seat and can use more cargo space on long road trips.

BMW roof box 460 (91"x32"x15"). Its on Page 10 item H in accessories catalog.

Thule and Yakima also makes something similar and may cost less than the BMW one's.