My first 1U server was not cheap or out of warranty - a brand-new first-generation Mac Xserve. I was horrified when I turned it on. It was too loud in a closet in an office with the closet door closed (no server room then or now.)
I made a box from recycled MDF (particleboard) which allowed the server to suck cool air from the back, below itself, to the front, and then exhaust hot from the back, above itself, to the front. There was a door panel that could be unlatched (on hinges) at the front, and one that could be unscrewed at the back. Air passages were large and had foam and/or carpet linings to adsorb noise. Made the whole thing some 6-7 times thicker and at least 6 inches longer than the bare unit, but it was bought for its abilities (and included software license), not its size, and I did not have anywhere to hide it that I didn't have to listen to it. It's now retired - the server is irrelevant age wise, and the power supply died (the server never overheated according to its monitors, just doesn't work right - probably bad capacitors - made it about 10 years or so.)
That box helped immensely. While you still know the server is running, the box sucks up a lot of the teeth-on-edge high-pitched noise from the tiny fans spinning madly. One (an HP, not an Apple, about 4 years old) I just picked up surplus is either going to go live in a room where I don't care about noise (not in existence when the previous one was bought) or I might consider a fan replacement. Problem being those are something of a mechanical nightmare and may involve fighting with fan-control software on modern server boxes. I could (and might) re-use the Xserve box, but it is rather ungainly (MDF is heavy - on the plus side, it does not resonate much) if it's not absolutely needed.
The box is effectively an "S" with the server in the middle of the S. The upper and lower ducts taper so the whole package is rectangular, with the server at a slight angle (back higher than front) inside. It could easily be altered if you needed a few servers in one box, or wanted to pipe it to air-conditioning directly or the like.
A fanatical job could connect the duct-work to outside air. This was not a fanatical job. I don't have any pictures handy and I'm replying to a 4 year old post, mostly because I'm sure it's a subject that comes up over and over again, so perhaps it will help someone in the next 4 years or so.