Reverse voltage circuit protection using an N-channel MOSFET
You've got that FET drawn backwards, and most of the time gate protection like this is done with a zener diode. 

You should make the 9k on your drawing into a 15K , and the 15k resistor into a 15V zener (assuming that 15V is enough to fully enhance the gate of your particular MOSFET).  If your MOSFET needs 20V to fully enhance it, then use a 20V zener)