Here's what I do in the desert. I bought some of these "cube coolers" from wally world and cut the holes in the top as shown:
Then put your carboys in them, and I freeze bottles of water and put one in each corner. I change them out daily. Works pretty well, especially if its already 78 there.
I also have a couple that I have cut to the size of corney kegs, and use them as keg coolers.