MA C3 Credenza frame is probably the best solution if you can get away with only 4U AND need it to slide out. You might be able to buy just the guts without the cabinet "wrap"? Might need a really good rep to get you to the right people to make that happen. Otherwise, you could pretty easily attach heavy duty drawer slides to just about any rack that fits in the space. I think 4U might be your limit though with most cabinet, desktop, or wallmount racks. Most have a "wasted" space at the top and bottom which eats up your Us. :)