I believe battery isolation switches were primarily fitted to rapidly isolate the battery in the event of fire on a competition car, and hence have a universal key?
If I were a professional sports car thief, its the one item I would carry in my pocket!
If theft is the main concern, I would look to installing a simple isolating switch which is wired to prevent the fuel pump from operating. Simple access to intercept the white/pink wire to the inertia switch that is mounted on the near side bulkhead adjacent to the coolant tank, would be a very simple and easy job to do. You could position the switch wherever you like, say under the dash with just a short run of wires.
