When I was in High School I sold a lot of my friends "Pizza Box" computers. I would buy a new Pizza box from a local Ma & Pa pizza joint. Take it to a place and have the cardboard laminated with 10ML Plasic to protect the Pizza Box and to strengthen it.
Then I would fit the MB and all the other components in it using custom metal brackets and screws. I would run it all on a modified external Power Supply with plug.
Oh yeah, I would also put a small galvanized steel plate on the inside of the bottom of the box to fit the motherboard to and to keep the box rigid.
My friends thought it was the "Coolest Thing" and I sold probably 40-50 of them during my 4 years!
It was a real hoot, I wish I had pictures... But just picture an Large size Pizza Box with the CD-Rom on the side etc.
I had a small business building computers at the time so I could equip you with the latest/greatest inside your Pizza box, or convert and existing system for a small cost!
Something like this, only mine looked very professional and wasn't flimsy. My P.S. was also external as mentioned above:
You would of made a TON of $$$$ if you got a patent for your creation and sold your pizza boxes world wide.
I was thinking about starting a website and making/selling them again. With my connections in the Food Industry I could get a hold of Pizza Boxes cheap that can be customized with Artwork...
I could also just sell the cases so people could build their own Pizza-Case PC!