I mistakenly ordered the thermostat without the piggyback cord preinstalled (which is cheaper than buying separate). Fortunately, I had a male bushing with wire clamp that matched the thread pitch and diameter for the thermostat to secure the wiring.

I was able to use the thermostat and piggyback plug with a small 500W desktop heater to keep my solar batteries warm in an insulated battery box that I built. With temperatures dropping into the single digits, the setup has worked like a champ!