Or another option, if you don't mind not being able to look inside the tank from the sides.
Recall your childhood science education and what happens when it gets hot. You sweat. The evaporation cools you. And when it's damp, you feel cooler. Just like the old "swamp cooler" idea of a large squirrel cage fan air conditioner that is fed with water drops.
So taking that idea, set basins of water around the tank and put towel around the tank, making sure that the towel is sitting in the water which acts like a wick, drawing the moisture up.
I found this explanation from a beer/wine making website. Pretty much the same concept because you are wanting to keep liquid at a certain temperature.
Everybody knows that as a liquid evaporates, its temperature drops. Why? Philadelphia refrigeration engineer Ike Ehrlich explained it this way: "Liquid evaporates in the presence of heat. But as it evaporates, it takes energy - in the form of heat - away from the surroundings. So the area very close to the evaporation will be cooler than the surrounding area."
It's why we get cold when we are wet. And why we sweat when it's hot. As the sweat evaporates, it uses this trick of nature to cool the surface of the body. You can use nature's tricks to keep your beer cool, too.
You will need a basin, bathtub, or even your brewpot, if it's big enough for your carboy to fit inside. Put a few inches of water in the basin, wrap the carboy in a big beach towel, place it in the basin, wet the towel, and let nature take its course. It helps to tie or rubber-band the towel in place around the neck of the carboy to keep it from slipping off.
The towel wicks the water up from the basin and it evaporates, lowering the temperature of your fermenting brew, depending on the relative humidity at the time.