Cardboard--it's inexpensive and readily available. What better use for it than housing delicate PC hardware? And while you're at it, why not instill terror into the hearts of passing children and pets by crafting a case into the shape of an evil teddy bear?
Bit-tech user Cheapskate might not be employing functional hardware, but his unnamed case-modding project is comprised of three interlocking parts, making future upgrades theoretically feasible. And yes, those eyes do glow a sinister red when the system is powered on.