Use the t-shirt trick. Find a bucket that your carboy will fit in plus a couple inches around. A bucket that comes up about half way is ideal as that helps with evaporation.
Put your carboy in the bucket and fill the bucket with water to around 3/4 full. Soak at t-shirt in water and slightly wring it out. The t-shirt should be wet but not dripping. Drape the t-shirt over the carboy.
If you can, place it where there is some air circulation (in a bathroom with the bath fan on or in any room with a ceiling fan or small fan to circulate air).
I've dropped the temperature of my fermenter 10-12*F below the ambient temperature in my place. But this is dependent on your relative humidity (humidity here is usually around 30-40%). The more humid it is the less effective the t-shirt trick is. Moving air through the room (an open window with a fan to draw air out) helps with high humidity environments.