"I know, cubes aren't exactly 100 percent original - but they're still one of the most popular and yet least done builds - and the smaller they get, the less of them you see. I'm sure a part of that is due to the difficulties inherent in fitting everything, as the cube shape almost requires the motherboard to be in only one of two places - flat on one of the faces (and facing inwards), or dissecting the middle.
The first idea is fairly boring and straight-forward. Take a normal case and give it a little extra width, and you're done! All the goods go on one side, the motherboard's back faces out and that's it.
Dissecting the case with the motherboard to put it on display is another thing entirely, though. Suddenly, there's a challenge in how small you can get the cube and still fit everything you need - a DVD drive, HDD and PSU get to be harder to fit. By the time you add multiple hard drives, card readers, LCD display and watercooling components, you get quite a challenge indeed.