Thinking like a thief, if I SAW a big safe, I would immediately make a plan to come get it.
Maybe that's a place to start. If a thief sees a safe, that's the first thing they'll focus on. Safes generally have valuable stuff inside.
However you don't need to worry about them taking and breaking the safe if they don't know it's there in the first place.
Sometimes a good hiding spot is as good as a safe. Not t say that you shouldn't get one. But focus on hiding it.