i travel with two pelican cases (until my canopy comes, anyways)...
i just put a lock on each one and then i have made up a small stainless cable with loops on each end. run one loop around one of the tie-downs in the back of the ute, loop it through the loop on the other end of the cable, run it through the handles of each of the pelican cases and then lock it up on the other side of the tub with another lock.
has worked for me. as others have said, if they really want it, they'll get it, but the locks and cable seems to be a worthwhile deterrent for the opportunistic thief.