rationalskepticism.org

I read a bit about the standard models. There are a number of materials that are very porous and thus would be very conductive to evaporation, on the flip side they may not insulate as well. Assuming you have a normal shaped pot you could increase evaporation by increasing the surface area. Maybe by making a wire frame you could drape the towel on to make a teepee like shape. This would have the problem in which gravity would pull the water down, though if you created a drip system that created an equilibrium so the towel remained damp all the way to the base without dripping I think that could work. If you're using batteries I'm pretty sure I can figure out a better way to do that.

The link above is an interesting idea. It looks like you need to use ammonia though. If you were to just buy standard refrigerant I think you could do more with it, I'll have to look into this more thoroughly.