Designing a simple fan + resistor system for heating air
Since it is the fan that will deliver the air via the exit port tube I would suggest that you will not want to vary the fan speed as that will cause a variation in the air flow out of the exit port. 

Instead you should setup a PWM driver to vary the duty cycle of the power to the resistor. This PWM duty cycle would be controlled through a feedback system that monitors the air temperature at the exit port of the box. This scheme allows the fan speed to be kept constant and permits a consistent air flow in the exit tube. 

One of the easiest methods of monitoring the air temperature at the exit port is to place a 1N4148 diode onto the air stream that is biased with a constant 1mA of current. Then measure the forward voltage drop across the diode with an opamp gain circuit and an A/D converter.