What type of equipment do you need to build one?
How do you use it?
What are the realistic expectations of using one?
In terms of brewing, I have seen folks keeping things cool by putting the carboy into a larger sized bucket of water, swap out ice or ice packs daily or more often if you live in a very hot place, and drape a wet towel around the shoulder of the carboy. For added cooling, you could point a fan at this whole setup.
I had a friend do this in his basement to brew a lager. It worked surprisingly well.