Because of the difference in thermal conductivity between glass and plastic, the wet t-shirt would be slightly less effective on a bucket but should still work. Also with a carboy you can have the shirt covering the carboy from water level to the neck, giving you much more evaporative surface area than what you could get with a bucket. If you are using a bucket, I would suggest dropping frozen water bottles into the surrounding tub to keep the temp down. Use enough of them and you can even maintain lager fermentation temps.
On the other hand, a wet t-shirt over a pair of jugs may actually raise your temperature.
Wayne
Bugeater Brewing Company
