First, it's important to use only refrigerators that have a metal interior, and no plastic on the inside. Heat can cause fumes to come out of the plastic, and you wouldn't want those on your food. Watch out for styrofoam insulation, too. The older refrigerators are best.
An adjustable electric hotplate or two can be used for the heatsource. Use drip pans, either placed on the bottom rack, or placed directly under the meat being smoked. You don't want juices dripping onto the hotplates.
Since the refrigerator is sealed well, use wood chips sparingly. Since it's not vented, a little bit of wood will go a long way in adding flavor to the meat.
Use a remote thermometer to monitor the temperature of the homemade icebox smoker. Do a test run without food the first time. Let it go for a few hours, until the temperature stabilizes, then make adjustments as necessary. That way, you'll know where to set the hotplate control when you're ready to actually smoke some meat or fish.
