Alternator Wiring with Kill Switch

Just run a # 8 or # 10 wire to the battery around the batt.cut off switch from the alternator. Put a 50 amp or better auto re-settable breaker on the wire. You can get it at Napa.
Put a boot on the alternator back, as it will be constant hot.
But 67dart273 is correct too. This just saves a $80.00-$100.00 4 pole switch purchase.
Lastly, a diode would have to be huge to handle the load. But possible if you can find one.
You could use a RV type solenoid (one that can handle constant voltage) that will activate from your ignition switch. But you still need to run a wire around the batt. switch.The solenoid will kill the voltage on the wire when the ign is off.