I've got a little grow on the go, probably similar in size to yours.
I'm running two 4 inch axial muffin fans, and they're working great.
I've used the same fans in different boxes, and never had problems
with them.
Jon's Plant Factory has them in Vancouver, but check
http://www.betterthannature.com/catalog/default.php/cPath/48_71
in the likely event that you aren't here.
The fans are pretty silent, have easily detachable power cords that come
in multiple lengths, and move a decent amount of air.
Hope the info helps.