Or a louder variation
Lightweight (to not harm boats or car when falling) but noisy.
Hmmm. How about small pillowcases loosely filled with jingle bells and a few empty soda cans (not crushed) and tied off? In addition to loose pingpong balls.
cable and padlock
I use a cable and a padlock. Like someone already said, straps are easy for any knife. There’s not much more one can do than lock it…if they’re really determined, they can just lasso the whole rack and rip it clean off the car.
Remember, criminals are lazy and fairly stupid. If you make it challenging for them, they tend to go looking elsewhere. Good luck
Motorcycle Alarm?
I used something like this when I had a motorcycle years ago. It worked pretty well. I’ve been thinking about getting another one to clip in the cockpit when I have to leave the kayak unattended.