I have never done this but I have seen it done. Place your fermenter in a larger tub, fill that tub with water until it is about a third of the way up the side of the fermenter and wrap a towel around the fermenter. Keep a box fan blowing on the fermenter. What happens is water will wick up the towel and the fan will cool it. I have seen this work when the air temp is 78 degrees. A simple stick on thermometer will let you monitor the fermention temp. It is a bit of work to maintian but you can expect to see a 6 to 8 degree drop from the air temp to the fermentation temp. There is a great youtube video of this here
Cheers mate and good luck.