When I cartop(truck campertop, really) my 12-footer, upside down, I run the ratchet straps over the top, and into the far scupper, up through the other scupper hole, then to the other side of the vehicle. That way, the yak is "captured" by the straps, and can't slide anywhere. How you attach the ends is dependent upon your vehicle. I've never had a problem, except losing a foot pedal once.
I'm an Eighth Day Adventist. On the Eighth Day, God went fishing!