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Chris, the better solution is to cool the mirror enough that there is no boundary layer, then switch the fans off while you're observing. I've never had any luck with high resolution observing or imaging with fans running, they always cause too much vibration.
Try installing several high flow rate cooling fans (eg computer fans, 80mm or 120mm or whatever will fit) blowing air onto the back of your mirror and have them run for an hour or so before you observe. Switch them off when you're observing so there is no vibration, and switch them back on whenever you're not observing (this is important, as the air temp will keep falling and you have to keep cooling the mirror).
This will not be perfect, your mirror will still likely be a few degrees C hotter than its environment, but it should be close enough to kill off enough boundary layer effect.
cheers, Bird
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