We built a shallow rectangular box, painted it black, glued on a clear plastic top. Before gluing on top we took a black hose and coiled it inside, so both ends of the hose were coming out a small hole at one end. Also used wood wedges to keep hose in place. We used a sump pump to move water from the pool through the coiled hose and through an additional black hose on the ground. With the sump pump on a 30 minute cycle timer, it helped raise the temperature.

Hope you get warm soon.