If I've had to park up with bags on the rear rack, I've used a pacsafe. Originally bought it for the bikes, but works as well on the car,

"Y mae dafad ddu ym mhob praidd"
1972 4/4, 1981 MGBGT, 1984 FLHT1340, 1980 XLH1000, 1990 ZX10