Not sure how much heat you want to move across the heat exchanger, but a simple coaxial (tube in tube) is pretty easy to put together. Just need a length of 3/4 inch copper, a slightly shorter length of 1 or 1-1/4 a few fittings and a little brazing.
Their length may be a drawback, but because of that, they sometimes lend themselves well to a system that is using convection in lieu of pumps.