I had a couple of lock boxes/car safes in the trunk of my last car that I wanted to use in my Prius. They both have cables that have to be looped through a secure area so that they aren't easily removed. I was concerned there would be nothing to lock them to in the Prius, but I removed the cover for the emergency fuel release in the cargo area (it is there in case you can't open the fuel filler door with the switch on the dash). Luckily there is a metal body piece there with a hole in it. I was able to attach a long-shackle padlock to that metal piece and then attach the cables to that padlock. It's not the most secure setup as I'm sure the cables on the lock boxes could be cut, but it would slow a thief down.
