The thermo cube on smarthome would be ideal but the temperature ranges they offer probably aren't optimal. I'd guess that I would want the fan to kick on around 100F .
The Winland thermostat might work if I could find a good deal.
Using an automation system is a little overkill for the project in my opinion, and I don't have sufficient wiring to the location anyways.
Although not the simplest solution, the bi-metal switches look like they are cheap and can carry the full 12v .5mA load so I could just wire the wallwart through it.