An ignition kill switch is more effective and baffling to a thief than a starter kill.
Any fool that ever had a starter solenoid go bad knows to bypass it with a screwdriver applied to the terminals at the starter, but finding the source of lack of igntion feed power is more perplexing and takes time, which criminals hate, usually being stupid and in a hurry
All you'd have to do is isolate a supply wire for any part of the ignition system that passes into the cabin, and shunt it through a 20 amp hidden switch.