Surely the kind that blows hot air doesn't have to be pressed right up against it? My space heaters are the non-air-blowing radiator kind, and I can imagine that they would need to be right next to the open fridge to have an effect.

But still, if one doesn't have such a heater, or can't wait 3 hours, the hot water washcloth works quite well and quickly, though requiring more manual effort.