As you may know, BMW and others, like Thule, make racks for your car.
If you want to make your own, consider using extruded slotted structural aluminum instead of tubing. You will be able to easily build a modular, custom rack for probably about $50 in parts. I use this stuff for work related purposes (mounting sensors, equipment, etc.) as well as a few home projects. It runs anywhere from under $1 to $2 - $3 a foot depending on size, etc. You can do an exact design and they will cut each piece to your specified length for a small cutting fee.
Two companies to choose from:
80/20 - "The Industrial Erector Set"
http://www.8020.net
Bosch -
http://rexroth.starlinear.com/framing_ec/index.asp