If you are not too keen to do a full cold-finger mod, you can also build a cold-box cooling, see
here for example. While it is simpler the results are inferior to a proper cold-finger modification.
Also, depending on the camera, usually the "sensor temperature" is not the actual temperature of the sensor but temperature of something else somewhere in the camera. For example, in cold-finger cooled 450D if the sensor is cooled directly to 0 C (measured directly on the cold finger), the camera will still report rising temperature, usually above the ambient. So the values reported by the camera have nothing to do with the real sensor temperature.