What I suggest is picking up a 110V STC-1000 unit off ebay and installing either in an external box, or better yet, directly into the door or side. They can heat and cool at the same time for when the temp fluctuates a lot and are small and cheap. I've seen them going for under $25 often.
The only downside is that they only display in Celsius. But it's quite easy to print out a conversion table to refer to.