I remember seeing a cheap simple solar heating system for a swimming pool. Make an angled simple frame work of a couple of telephone poles with a cross member at the upper top of the two poles. Then steel barn roofing or siding dark brown that is slanted at an angle facing south with the bottom of the siding draining into the pond. No glazing. Pump pond water across the top and let it drain into the pond. For a small pond the size of the siding might be 20 by 40 feet.