Well, you could do what MetalHeadMike does and use a stove pipe...
MetalHeadMike":2mgj8t8m said:
Seriously, though, if I was going to mount a fan in the back, I would put a 4"-6" panel-mount fan mounted directly to the back blowing outward placed close to the center of the tubes and close to the top of the shelf. I picked up a couple of low noise fans for my rack from Sweetwater and I can't even tell they're on if I'm more than 2 feet away. Can't hear them at all from the front. Don't remember the price, but it seemed reasonable. Computer supply stores should also have them, but you'd probably have to change the connector to use it with a standard outlet and make sure that it was 120vac unless you want to run ten 12vac fans in series.