Another vote for the plastic tub method. The advantage over a bathtub is that you can fill it fuller -- I'd recommend keeping it just an inch or two below your beer level, so you can maximize heat transfer from the beer to the water in the tub and minimize temperature fluctuations. (Also, those liquid crystal stick-on thermometers don't really like being submerged. If you mount it high and horizontal, you can avoid that.) I found that even without adding ice, the temperature stayed 2-3F below room temperature just due to evaporation. Since RT in my house is about 67-68F, that drops it to about where I want to ferment an ale.
All that said, I love my new fermentation chamber. I think I will love it even more now that I have a dehumidifier for it so it doesn't turn into a mold factory.