You can make the exit of the box have some louves pointing down, like window blinds. You can also mount the fan to something vibration absorbing. Mount the fan to a small square of wood, then mount the wood to some rubber sheet or tire innertube then to the inside of the box. You could also take a screw and thread it into the wood where the fan would be, then hand a 1 or 2 oz fishing weight off a rubber band. Would absorb any vibration causing sound from the box surfaces. Just piling some dirt on the side of the tube might quiet it down too.