On my road-going DRB, I placed the switch (marine type with removeable key) on the rear bulkhead, just inside the passenger door latch. The DRB has the battery in the engine compartment, just behind the passenger, so this spot for the switch kept the runs of monster cable to a minimum.
I can easily reach it from the driver's seat, & kills everything except the alarm/immobiliser. With the red key removed, it is quite hard to spot the switch body (small black bezel set against black carpet).
Regardless of where you put the switch, it would be very easy to add a Bowden cable setup to flip the switch off from any other remote location.
Kind Regards,
Peter D.