well. being that water holds temperature much longer than air, I would suggest getting a cooler, putting it in that special spot.
put your fermentor in the cooler, fill with 19 degree water, soak a shirt in that 19 degree water, put it over your fermentor (assuming it's a carboy or better bottle) and let it sit there.
the water should reach 20 to maybe 21 during the day and it should maybe reach 17 at night. it should even out the temperature a bit more. use a fermometer, because if fermentation picks up nicely, temperatures should rise (though at 64-68 I don't think fermentation would be rolling)
the shirt would be kind of like a wick inserted in the water, hopefully covering the carboy in that same water.
just a thought.