Forget about copper tubing. Copper is so expensive now that thieves would literally rip it off your roof just to resell it later. I work for a company that sells industrial metals. We have folks call all the time attempting to do what you'd like to do. It all boils down to - doing yourself is too expensive. Buying a ready made rack from a manufacturer that is designed for your car is the only way to go. It's a whole lot cheaper and more efficient.