Maybe you should consider a cardboard case like the one below?
Sounds odd, but when you think about it there are many things that you could fit a PC inside. All you need is an ATX PSU (Power Supply Unit) and a switch off an old power supply (ATX style only), or you could make your own Switch/Reset using momentary switches from Maplin or the like.
Cardboard PC Case
Construct your own ATX PSU switch
If you fancy creating your own ATX switch and reset button, it's easy check out this site.
Apple Mac G4 PC Conversion
Maybe if you have an old G4 lying about you could convert the innards into a PC!