If you followed David Murphy's path to building a budget PC with a cardboard chassis, then why not compliment it with your own homebrewed Surface, also with a cardboard exterior?
Microsoft technology evangelist Paul Foster posted a YouTube video showing how you can build a functioning multi-touch surface using budget parts. Items you'll need are paper, scissors, picture frame with glass, tape, cardboard box, a webcam, and multi-touch software such as Touchlib.
From start to finish, it takes Foster less than four minutes to complete the project and run a short demonstration. Of course, that's with a cardboard box - skilled modders will want to invest a bit more time coming up with custom enclosure.