Dave, I was thinking that repurposing a clutched fan from a small automotive application would work. Perhaps a water pump would also work for the bearings and mount? I don't think it would hurt to run one dry.
TPXX, that was I that made that suggestion, but after it was made I went and looked and all the attic fans are pretty big, about 10x bigger than needed to cool a Listeroid. If you do a search on snap switch on this board, you'll find where you can get some thermostatic AC switches that will work great for a small ac fan.