For a cheap heating source you should look into flex watt heat tape. It is available in 11" wide and you purchase it my the foot. It is 20w per linear foot. You do have to purchase a thermostat for it or you can use a dimmer switch with it. You do need to be cautious with it around water, use liquid electrical tape on the connections to prevent issues. It costs about $4 per foot and you need a cord to hook it up. I purchased mine here...
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