Ok the ciruit is ready! Hopefully you can follow my crazy diagram and explanation
R1 and R2 form a potential divider, R1 being a thermistor and R2 being either a carefully chosen fixed resistor or a 'preset' variable resistor (the type you change with a small screwdriver). As the temperature varies the voltage to the gate of the MOSFET Q1 is varied, thus causing the resistance across the source-drain path of Q1 to vary. As Q1 is in series with the load RL (fan, peltier, etc.) this varies the voltage across the load.
The whole thing costs next to nothing and is pretty simple to assemble, the only thing to watch out for being the temperature of Q1. Q1 can be any N-Channel MOSFET of suitable power.