Adding polarity protection to this circuit
Your circuit may not need 'protection' as such. If polarity is reversed the FETs will simply conduct through their body diodes, causing the heater to turn on continuously. The FETs will get hotter than normal, but hopefully the user will quickly disconnect the battery after noticing that the heater has immediately turned on and the switch has no effect.

To make it more 'idiot-proof' you could install a loud buzzer across the supply input, with a diode in series so it only sounds if the battery polarity is wrong. When the user tries to plug a battery in the wrong way around the buzzer will go off.  

I would also use a polarized battery connector. If someone wishes to use a battery with non-polarized connections then they will have to make an adapter cable to match your connector, and they will be responsible for making sure the polarity is correct.