Hi,
if PWM is not an option how about using an NTC resistor in series with the fan?
The resistor will reduce both voltage and current, but is the easiest way to vary fan speed with increasing temperature.
I assume that's what you want.
Use a basic resistor value of the same resistance as the fan motor. Under normal conditions (temperature 25deg/C) the fan will run at 1/2 of the supply voltage, resulting in very low fan speed, and increasing with increasing temperature.
Boncuk