I spotted the The Wrebbit Medieval Clock kit in a store a couple of years ago. I was tempted to buy it, but decided not to since at the time I had a half-built wooden clock project to finish. I also wondered if a cardboard clock would work. After seeing this video, I now know that it runs. I'm not sure how accurate it is, but that doesn't matter to me very much. This would be the next stage in my self-education. It's also just beautiful to watch. Now that my wooden clock is done (and working), I can reconsider this one!
From the product description:
Build your own medieval clock, and it works! Discover the fascinating clockmaker's art. Craft kit, easy to assemble. Finished clock measures 13 1/4 inches wide, 21 inches tall, 7 1/2 inches deep. This magnificent clock, rooted in medieval art, is a remarkable tribute to ingenuity and precision. Though it is made entirely of sturdy cardboard, it actually works and tells time without fail.
Here's where you can order your own Wrebbit Medieval Cardboard Clock Kit. Apparently, they make more than one model of carboard clock as shown by this assortment Wrebbit Paper Clocks available on ebay.
P.S. If you really want a challenge, you can forget about the cardboard and try Paper Clock Book With Pendulum. That's right, a clock made entirely out of paper!