Camper fan speed control

During our last camping trip we really need to control the exhaust fan over the stove,  it was really cold out and the fan was a bit to strong. So I came up with a couple ideas:

1. Use a resistors to drop the speed. The problem with this is the power loss. Don't forget were on batteries and do no wish to dump energy as heat.

2. PWM - this would be the most efficient use of power and would not required expensive components.

Here is the schematic:

Used vero-board for this project since it was so small

Frequency of PWM is out of the range of hearing so it's quiet, I originally tried 1khz PWM frequency on the breadboard but it made the motor sound like a screeching cat. So I made C1 smaller until i could no hear the PWM.