Anyone buying a thermostat for a homemade incubator, I highly recommend looking up "STC-1000" on Amazon or Ebay. They are usually around $16 and are FAR better than anything you will find at the home center. The only other inexpensive thermostat I would consider is a wafer type from an incubator supply place, but I bet that actually will cost you more than an STC-1000. Wiring them is easy, I put it in an electrical box outside the cooler and run the wires for the heating element through the drain plug. That saves space inside and prevents any issues with electricity meeting high humidity. I use this on a very large cooler that is my hatcher and it hatches 90 - 100 chicks a week for me, some breeds hatch nearly 100%, so the ones that don't are not likely the fault of the equipment.
Make sure you get one for 110v, they also come in 12v and 220v versions, but you probably want the 110v version.
Here is a link: http://amzn.com/B00BMLCGF8
Make sure you get one for 110v, they also come in 12v and 220v versions, but you probably want the 110v version.
Here is a link: http://amzn.com/B00BMLCGF8