It basically consists of a float valve where the water level isand as the water evapourates it is replaced by fresh water supplied and powered by gravity. Hopefully there are some photos of it here (using my iPhone to blog this for the first time).
Construction consists of acrylic cut to size. It is held there by a clamp (so it is easy to adjust if needed). It has been running for a few days now and my water level has remained constant. Quite impressed with my handywork actually.
