My car has a number of holes in the roof of the trunk (there's a crossbar with punched out holes in it). I bought some 1.5' lengths of the thickest, yellow/rubber coated chains I could from the local lowes and a few steel eyelets, fender washers, nuts, and a set of 4 fairly cheap keyed locks (I think they were $8 total for the locks). Drilled holes in the side of the box to accomodate the eyelets, sandwiched the wall between the washers and the nuts on each side, threaded each chain through the holes and didn't leave much slack. I had one stereo stolen, I figure that the theif would have to have some serious lock cutters or a bunch of tools to get the box out quickly.. it's also chained at the very back of the trunk, so it's difficult to get at.... more than a 2 minute grab. I did the chain thing so that I could get at the spare tire without much difficulty if I ever needed to.