My roof rack and bars are Thule ones, so they are all locked in position with the same lock barrels. Now, they aren't up to much, because the barrels are mounted in plastic, so shoving a screwdriver into the lock and turning it hard enough will break the lock.
I've got one of the lock loops with a ball on the end, so I loop it through the wheels and frame, and lock the ball inside a door, and it's sort of secure, although the cable can obviously be cut moderately easily with suitable cutters.
When I've left the car I can put a chunky D-Lock (Kryptonite New York) on the bike, but obviously that's just locked the bike to itself.
Is there anything else I can do to make things more secure? At the moment the bikes I'm putting on there are (relatively) cheap, and grubby, so look less desirable, even if they would cost me a shitload to replace. I don't think there's any easy way I can sensibly secure things any more, but I'm curious if others have any additional suggestions.
(This might be more appropriate in Vroom, if a moderator thinks so, feel free to move it).