I just finished a chest freezer conversion and tried to do the build on as tight of a budget as I could. I've seen alot of posts about using thermostats and such to make your own temperature controller rather than buying one. I took it a step further and created my own controller circuit. I've had it up and going for about a week and it holds the temperature at 40F within about 1 degree. It's an anlaog comaprator circuit with an LM339 chip. To my knowledge a 741 Op-Amp could be used.
The schematic is pictured below. It doesn't show the comparator power connected, but it runs off the same 5V as the other components. All total, the build cost about $25 (estimating for the cost of pieces I already had) and if you can solder, you can build it (as an ME, I only possess passable soldering ability, but it works). Ideally, the resistances would be bigger, but I tried to use parts I had laying around to keep the cost down.
The parts list is:
5.1V Zener Diode
Thermistor (~200Ohm at 40F)
I would've liked to use a thermistor with a higher resistance at 40F, but I had the one I used already. Also, some of the resistances used were created by combining resistors to get the desired behavior from the circuit.
Eventually, my goal is to combine a power converter onto the same circuit board as the comparator and thus use only one power cord instead of two (I will get power to the board from the connection that used to power the light in the lid of the freezer.)