Posted on another thread as well but in addition to Kanulock strapped up down and left to right, I also wrap each of our waterproof bags in a pacsafe net which also locks to the rack. That way they're further protected from theft e.g. being cut open and provide additional securing points to the car.

https://pacsafe.co.uk/collections/p...s/120l-anti-theft-backpack-bag-protector