I am using this adapter to use extra C batteries I have laying around in our baby swing which uses D batteries. Plan is to use food aluminum foil to make a contact between positive terminals of battery and rely on using spring in battery holder to make a contact with negative terminal. UPDATE 12/24/14: I have changed dimensions of positive side a bit, making it easier to keep foil in place.Cap with smaller inner hole is for positive terminal side. Stuff household aluminum foil in there to provide contact between battery and battery holder positive contact. For negative side just use as is since most battery holders have spring in place which should touch C battery negative end.