Back in the days when I was running my Sun Netra T1 Server pizza box I needed a place to put that baby. As it also was incredibly noisy I designed a server rack that would double as a simple sideboard.
Closed server rack
The basic idea was simple since the Netra drew in cool air in the front and exhausted warm air in the back. As I did not want the noise to have a straight path trough the rack I settled for a s-shaped air-sound path. on the back cool air was drawn into the case to the front. A wooden divider board basically made this act like a duct. In the front (where the rack rails are mounted, the divider ended with the rails. 
The gap between the divider and the door allowed the server to draw from this cool air eshausting it at the back where the principle is mirrored – warm air at the top goes trough a duct to the front of the cabinet where rubber seals effectively seal the door.
A plexi glass on the rails filling up the unused space made sure that warm and cool air did not mix. 
I installed a second door on the side for easy servicing. A separate shelf was installed as well.