I have to wonder if the dimensions are similar to this bag (
http://www.ebags.com/ebags/mother_lo...modelid=125549.
One difference that I pick up from the customized luggage photos is that they appear to taper in depth towards the top which would allow for the roof cover to close over the luggage. The height of the bag appears to be around 25"-28" based on the photo showing it alongside the hood of the Z4.
These cubes may be an option as well.
http://www.ebags.com/ebags/slim_pack...modelid=107842
Just throwing some thoughts out.