G'day Dave,
One method I use with soft bags is cable bike locks, you can get "uncoiled"/"straight" ones which you can feed through the boltway and out the loading port, then feed the tail back through the eye as per the attached photo. The other end hangs out the zip of the soft bag and can be ganged together with several more to another cable looped through a solid spot in the car.
It's a tight fit through a rim fire action but you can do it.
Cheers
Dave Groves