I have added a 3' length of PVC to my subterranean pipe. They make inline fans for boats that fit right over the pipe, but they draw a lot of amperage and move a lot of air. I don't need much, but I am only finding cooling fans for computers that will require adaptation. What can I use that is somewhere in the middle?
Update 11/12/12: I meant to return to this post and let everyone know that I ended up getting a 4.75W fan at Radio Shack. I wired it directly to a 5W solar panel and it is working beautifully. Look at later posts for more details.