I want to do this myself. It's easy to find the parts and will (I'm sure) work like a charm. It's Alton Brown's design: Start with an oversized terra cotta clay pot with a drainage hole in the bottom large enough to thread the plug of a portable electric burner through. The bottom diameter must also be large enough to accommodate the burner with a little clearance around the perimeter. Terra cotta is used for all kinds of baking containers, so it's insulating properties are ideal for smoking.
For the cover, find a bowl-shaped terra cotta planter with a drainage hole for the barbecue thermometer (make sure you get one with a range down to 85-95 degrees). Be sure the bowl-shaped planter's top diameter matches that of the bottom and the top sits securely on the bottom. Get a sturdy metal pie plate to sit on the burner and hold the wood chunks of your choice. And finally, get around grill rack that just fits in the upper portion of the first pot (this is where you will place the item to be smoked.) The tapered sides of the pot will hold the grill rack firmly in place. According to Brown, these items can be purchased for approximately $50 - a bargain compared to a good pre-built smoker. And the electric heat source means you'll be able to regulate the heat accurately and without having to feed a wood or charcoal fire throughout a long smoking time.