We make them out of old refrigerators. The older, the better (the tar & paper-insulated round cornered ones from the '50's are awesome, but are becoming collectible so perhaps not cost effective) but any will do.
Converting a 'fridge to a smoker means punching holes in it, so kids can't suffocate should they somehow get inside one, but it's a good idea to put a hasp and padlock on the door to keep the Nanny State from harassing you.