These straps work fine. http://kanulock.com/
I've used them on several road trips, with the boards living on the roof of the car for a couple of weeks, parked outside dodgy motels and camping grounds.
Most thieves are opportunists. They take stuff because it is easy to take. One good tug on a locking strap and they will get bored and move on to something easier to steal and easier to sell.
My only concern is a pissed off thief vandalising the board. For day to day use I keep my boards inside the car. The Sector 60 fits in fine diagonally in the back seat with a short surfboard and/or a TT on the top. I throw a piece of black fabric over the boards so they can't be seen through the tinted windows and in the dim light of the car park at work.