Friction is a function of surface contact area, force and the coefficient of friction. When I car-top my kayaks, these three variables are what I have to work with.
Ratcheting up the force is the simplest way to increase the holding friction. However, this can strain and stress a boat, an expensive investment to damage!
Commercial racks increase the surface contact area with special cradles. Unfortunately, these are expensive and reduce the general function of the rack.
Sleeves of non-slip rubber to augment the friction easily secure my boats. It's cheap and general, rather than brute force or elaborate racks. A more simple solution.