Go to any marine store and ask for a battery disconnect switch.
I don't care for the separate key as it can get lost. As long as the switch is in an enclosed area, not much chance of someone messing with it (if they break into the compartment, they can pretty easily wire to get past the switch).
I also wouldn't wire the landing gear separate. I'd rather have certainty that the power is down for everything, if I'm working on the system. Since it's so easy to flip the switch on if I need to run the landing gear, I don't see much benefit.
