if i were you i would try pats water idea, i know from dealing with greenhouses that they put in ponds or make a wall of black bags filled with water with the idea that the sun heats the water in the day and the water heats the greenhouse at night. or have you seen the things they put around tomato plants called "wall of water" its the same kind of deal. if you boil water before bed and put it in a jug or bucket, it should keep the coop warm until morning
