Another inexpensive solution is installing a water heater. Remove the insulation as best you can and set it up in a loop of your choice of waterline a few inches from the edge buried a few inches powered by a small circulation pump available online for about $25. If needed you can add a car radiator with fan or use a barrel for more volume but at zone 8 most likely not needed. Your choice of electric or gas.
 I am zone 5 and also have 2 layers of bubble-wrap between 2 poly layers. You can get a 4' x 250' roll for about $50.