I think there's a huge waste of heat under the cables. Why not buy insulating panels -styrofoam - in the building industry, about one inch thick. Another suggestion : the heat will be concentrated above the cable. I would bury it in half an inch of sand. A thermostat looks compulsory unless you check the temperature constantly and are ready to switch the cable on and off 24 hours a day. 500 watts is impressive when a 60 ft long gardening heating cable only burns 90 watts...
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