This has been covered a fair few times on this, and similar forums.
I have fitted fans on 6 different occasions now. One or two computer fans zip tied to the inside of the top vent, small (iluminated) switch inside fed off a nearby 12v supply that is dead when the master switch is off (if possible the lighting curcuit which ensures the battery doesn't get drained over a long period if left on in error)
Total cost about £15 and a couple of hours of my time.
I have never bothered with a thermostat as I cannot see the point (that's why I use an illuminated switch, it acts as a "reminder")
No problem over-cooling when its cold, the switch just stays in the off position!