If you have a roof box, put all the crap you'd usually put on the boot in there, and then put your bass on the boot?
However, when our kids were little, and we had dogs in the boot, our roof box was always full of all sorts of family holiday detritus, securely strapped down, but just normal family luggage. We could lock it (which we did) but there was no real reason why anyone would target a roof box full of dirty laundry (basically) over some of the more easy targets.