i had such an thing in mind myself a year ago. wanted to pack a G4 into a little suitcase with a TFT built in the lid. had made some plans already and i know it would have worked. but i was too damn lazy to actually do it...
i think this wouldn't have been much more difficult than fitting a Powermac in a drawer or the like... and could be powered by a car battery (so much for portability)
but i wouldnt go for the cube. it is a very tight packed masterpiece already. could turn out being difficult to get it into in any different shape than a cube