Even a bathtub full of cold water will do the trick, if you have one to spare for a few days. I advise dropping the $10 on a small tub now. I have tasted beer that has been fermented too hot, and beer that has been fermented at the right temperature, and the difference is well worth spending an extra few bucks. We are talking about the difference between something you drink because, well, you made it, and it shouldn't go to waste, and something you want to share with your friends because you can't believe it came out this well.