David I use the 150qt marine cooler and I don't notice a huge difference in humidity levels between the top and bottom. I use two, 1lb bags of Heartfelt beads, one on the bottom shelf and one on the top. I also have a digital hygrometer at the top and bottom of the cooler and while they are close to the beads they are not right next to them. I keep them calibrated and they are always within a percentage point of each other.
I know there are a lot more experienced people who say fans make a difference, but that's just what I've seen so far.
