I picked up some of those "dish" tub things from smart and final. They are only a few inches tall, which allow more air flow around the unit. The best part of a true swamp cooler (I grew up with one) is airflow and evaporation. If you put it in too deep of a tub, then the air cannot get in as well. I can drop the temp ~10* or so from just tap water + towel in a shallow tub.
Also, I dont wait for the water to "wick" to the top to start, I dip the towel into the water and pull that up and around the carboy. This gets the cooling to start right away. In Arizona, where its dry.. Doing this once every day or 2 is enough to keep it really cool. My local LHBS had some pump deal with tubes and holes in it, but I think thats a little excessive unless you don't watch it at all.