A simple and effective mod to this would be to put a baffle to separate the cooling chamber into two chambers, and leave about 2-3 inches of space between the bottom of the baffle and the bottom of the chamber. That way the air flow will be over the first ice container > underneath the baffle > up and over the second ice container > then out to the fermentation vessel. As you have it, the air is most likely not coming in contact with the ice and cooling down.
I made one of these bad boys a few months ago and it works great. I am having some issues with air leakage, as I didn't completely finish the weather stripping, but I just throw a warm blanket over it and its good to go.
