I found a limited selection to get a box that will clear the rear deck spoiler when it's open. That is, if you want a box long enough to carry skis. Wound up with Yakima Pro Series 12. But I don't see this style currently on REI.com. I bought it from a local REI store, with the assurance that if it didn't fit (clear the rear hatch) I could bring it back. I mount it to standard Thule cross bars (non aero). Mounts solid. I don't know that the Yakima box mounts are any more or less secure than Thule. I do know that I have driven into strong headwinds and that it hangs on at effective wind speeds approaching 100 mph.
2005 X3 3.0i, CPO, white, auto, adaptive xenon lights, stock suspension, premium pkg, cold weather pkg, heated steering wheel. Self installed: factory hitch, paint protection film, mud flaps