Kenny-
I've had good luck with the following. I went to Home Depot and bought a bathroom sink cabinet, the typical sized small one, seems like it cost around $60. Then I cut off the top and bottom so that it is about 26 inches tall, give or take 1/2 inch... depends on your desk.(If you're putting it under your desk) I put bedding type of foam inside that I got from Walmart, sadly I could only find pink, and it has the pointy egg-carton type of surface. I spray painted it slightly gray so I wouldn't be quite so ashamed of the pink, but it was only moderately sucessful.
The cabinet has a door, I reverse mounted it since I wanted my computers under my desk on the right hand side. On the rear, I cut some of the wood for a horizontal door that swings downward, mounted with a piano hinge. I put some sort of eye-hooks to latch the top shut when desired. I also cut a hole for a fan, which I've never bothered to mount, and there are 2 large holes in the bottom back of the cabinet for cable pass-throughs... to make them more sound proof, I cut circles out of a heavy foam rubber door mat.. or it might have been a place mat, doesn't matter, it's about 1/4 inch thick. Then I cut slits in a * shape so that when the cables go through, it springs closed around them. The rear door, I only latch shut for maximum silence, as when I'm recording a solo acoustic guitar or something, where I also close the front door.
Heat hasn't been too much of a problem, though you don't want to forget and leave the thing closed up for real long periods, but I'd say I've left it shut for at least an hour or so with no real heat buildup. For some reason, a couple of times, the heat did build up, so keep an eye on it if you try a design like this.
I sort of based this on some of the pro designs I've seen from Raxxes... (sp?) but their prices are insane, and this works quite well. I stained the front door a medium light cherry, which matches the stain on the desktop I built from Red Oak plywood, also stained cherry. I built the desk in a pretty nice way..including raised platforms for my Tannoy monitors, etc. The sides are 1/2 inch plywood painted with a black paint from Home Depot... with sort of gray specks. (Note: don't use 1/2", go with 3/4.. I used it because I already had it around, but it's pretty minimal, not as rigid as I'd like) But the whole thinglooks pretty sharp and is real sturdy. The pics on the site I'm sending you don't do it justice. On the opposite side from the computer iso box, I installed a pro rack metal "cage" which I enclosed the same way, so these 2 boxes act as sidecars, and the desktop lays on top, but slides into slots... I'll post some pics here so you can get the idea.
http://pigstilts.deviantart.com/scraps/ In some of these pics, you'll see that the monitor "bridge" is too high, I miscalculated, but fixed it. Also, in the final pics, my JBL 4313B speakers are on the Monitor bridge, but I plan to move them to the rear on stands. They're not in use right now, and I think they're distorting my bass response. I also put the video monitors a bit too high for comfort, I have a neck injury, so contrary to the "eye level" principle, I now plan to move the video monitors a bit closer, lower them down a lot lower, and slant them backward, which should make my life more bearable. You'll notice I also put some rack space below the monitors on one side, the side with the computer cabinet was too wide for racks, but I can easily fix that should the need arise.
I hope this helps. I'm still learning as I go, so my next step is better bass traps, etc. If you have any questions, you can email me at dcoyer@comcast.net. I could send you a couple of digital pics of the rear of that cabinet if you need. (I know I gave a lot more info than you asked, but that's me for you. Good luck, Don