I would start by looking at small heater cores as used in very small cars and pickup trucks, I once saw one that was about 5" x 7" and only an inch thick, which I believe came from a Datsun sedan. Fellow had it on a small gas engine he had built, looked really nice there. Also, check out your local computer parts house such as Fry's electronics, they have water cooling systems for computer have the neatest little 4" x 4" radiators you ever saw!