I wondered about this too. I have a 9' 4" and for the most part boards don't get stolen locally, but I surf spots where "non-locals" come from "the big city."
Anyway, I have a pickup with a canopy, but the board won't fit inside, so it's on the roof. I don't worry about an 18' sea kayak on the roof because it's just too big to haul away without a rack on the culprit's rig.
But the board is an easy target. Cut the straps and toss the board, bag and all in the car and they're off -- it's gone. Or if they're bold, unleash the straps and steal them too -- Dakine and nice.
Anyway . . . seems nothing you might secure the board with would slow anyone down much. A bike lock on the bag, and it just gets cut loose.
So I wonder about a secure "cartop carrier compartment" -- like the streamlined boxes you see on the roof racks for luggage. I wonder if they make them in a length for a board. If they don't, someone should.
The other option might be to cobble together a "strong box" from whatever building supplies might be available. I can do a water-resistant box from various wood products, plastic counter-top, etc. so that it wouldn't fall apart in the rain.
Contractor suppliers might have some sort of box that would accommodate a board. Contractors need to secure equipment on trucks, and some of it is large.
And so, bolt that to the truck, and put a lock on it. Seems huge, bulky, inconvenient. But not as inconvenient as having a board ripped off.