i hope youve put the bolts through the chassis member, not just the boot floor as there not that thick metal, wouldnt take much to force it off the floor, and if they like your car they may trailer it away my old mate has his astra gte taken this way.
I have a small strongbox that fits under the front seat of the car, attached by a locking metal cable similar to a bike lock. Yes it has its weak points but given that the glove compartment in our car doesn't lock and any thief would have to break into the car past the alarm system to access it, it's better than nothing and better than leaving items loose in the car or in the tent. It's just big enough to hold passports, tickets and cash, not electrical items but it is very economical on space. . And when we don't want to use it it can be easily removed from the car once unlocked.
My boot lid is isolated from the central locking of the rest of the car. If a lout broke the window to pop the locks up, the boot remains locked and needs a key. All in all, my pot noodle is probably safe.
So is the safe just incase someone does get the key ?????????????
I've found that umclipping the back of the rear seat (just needs a good tug) usually reveals access to the boot once the boot trim is pushed off. Then the boot lock can be popped and boot opened and safe could be quickly dealt with using a cheap plasma cutter (or an arc welder with a carbon electrode).... :)