Get four toilet plungers and cut the dowels in them to the height that the roof rack needs to be. Run a screw through the plungers where the wood is inserted into the rubber (to make sure they don't come out). Get some copper elbows and copper tubing the diameter of the dowels. Build the rack the size and shape that is desired, and place on the car's roof with the plungers resting on the roof. Tie down at the front and back of the car (to something under the front and back of the car).

I don't know if this will work with the car in question, but I think it's a good approximation of how some store bought car racks work.