I use the same... except without the fancy cord.
I took a $1 harbor freight extension cord and cut it in half. Male end goes to wall, female end gets the freezer's wall plug. This way, I didn't have to cut the freezer's cord.
If you want inexpensive, there are some line voltage thermostats out there that will control to 40°F. If you want a lower temp, you'll have to use a temp controller like the above Johnson model.
If you _really_ want to get cheap, you could rig up a thermostat from a junked fridge.