Why wouldn't a gas spa or pool heater work? Pardon my stupid but I don't have N.G. available so I have to rely on electricity for heating my outdoor hot tub, that's not particularly well insulated, and it only adds about $40 a month to my bill even with the circulating pumps. Of course even though it's pretty big for a hot tub, about 400g. it's no Lake Pete, but at least you're in out of the wind and weather.
Don't know how affordable your N.G. is but electricity here is about to over take plutonium. Boy I'm glad we went to deregulated utilities here. Welp, gotta go. Have to go make sure the kids are keepin' up on their quota of coal for the day.