I use a standard 6-foot cable, the braided flexible kind with a loop at each end. Pass one loop through the other around some locked part of the rack and pull it up tight, so you have almost six feet of cable to work with. Thread the free end through the wheels (I go through the frames, too) and around a crossbar or tower on the other side of the car and lock the loop to the standing part of the cable. If you have slack in the cable, you can wrap it around a BB or whatever's handy.