A temp-controlled, 12VDC fan is pretty easy to build. I have a couple of them in my equipment rack and they work exactly as designed. I guess that's the benefit of making things yourself, it may take longer, but your finished product works exactly how you want, as long as you are willing to put in the time to research and think about what you need.
There are numerous places on the web to find temp-controlled 12VDC fan circuits, seek 'n ye' shall find :-) I modified my design from what I found (adding switches and additional fine-tuning circuits) and I never drew it up or I would share with you. A 12VDC, 120mm fan works very well, as the RPM can be kept lower than an 80mm fan while moving as much, or more air. Just be sure your temp control/power circuit can handle the current required by your fan, which will probably be around 150mA. A small, 12VDC wallwart-style power supply can be used to power everything and decent control circuit that uses pulse width modulation (PWM) to control fan speed is actually quite efficient.
Construction time took me a few hours, of which at least 25% was for testing my add-on circuit and tuning for proper temp adjustment to start the fan. Everything cost me less than $12. Don't forget to get a guard for the fan where it may be touchable by kids or pets, as that could cause unfortunate problems if Jr. sticks his paw into the rotating fan blades.
You may also find other uses for this hardware whenever you need a temp-controlled fan. It's handier than I ever thought and I'm glad I built them.