There's only 2 major parts to any case. The frame and cover. Of the 2, only the frame is of any importance as it's got to have enough rigidity to stand on its own without support from the cover, and also has to be able to accommodate the components. Unless you are very good at engineering, fabricating etc, I'd strongly suggest you get a frame pre-built, as in a cheapo case off ebay, scrap the cover/plastic, then get as creative as you please on the new cover. You'll just need to research exactly what piece of junk case has the component layout (top or bottom mount psu, hdd bays etc) since you'll also be able to change stuff like riveted hdd bays/optical etc.
If you wanted to get really creative, you could do stuff like get 2x identical mATX or small form factor frames, weld them together, and use a full ATX mobo in 1 with the psu in the other, like the Corsair airflow cubes etc. Totally hidden wiring, hdd, optical etc.