How to override an attic thermostat with a wall switch?
OH!  That's easy.  


Get a 3-way switch.  Common goes to HOT.  
Traveler 1 goes straight to the fan.  
Traveler 2 goes via the thermostat to the fan, so that line is interrupted by the thermostat.  
Traveler 1 and the switched line from the thermostat merge with the fan's HOT input.  


Neutral needs to be part of the wiring bundle, so use 12/3 if you're not using conduit.