While technically a vHack, this is a design that I've kinda had developing in my head for a while, and hope to build someday. It got started with thoughts of a Taco CC upgrade, but much as I like the idea of a G4 mobo in a CC case, hacking up the plastic as some have done to get it working is more than just a bit distasteful to me.
So I decided to put the CC concept in a more modern design, and with more modern components — and a bigger display! What I came up with does look a bit like the G5 iMac, but uses off-the-shelf drives and display, and components pulled from a G4 Quicksilver (it could also accomodate the innards from any of the other G4's as well. It all depends on what's available when I finally get to the point of building it!)
As for construction, I'm thinking a mix of aluminum and polystyrene... Aluminum where structural strength is needed, and polystyrene elsewhere.