These have been kicking around for a while now but a friend just discovered them so I made her one for her birthday. This one uses a variant of the the living hinge design I saw on the MAKE blog. It uses a standard size servo, I removed the controller board, patched directly in to the motor and modified the servo for continuous rotation. It is powered by 3x AA batteries so it should last a while, which is convenient because I got too excited and glued the box shut before I figured out how I was going to change them. If I make another I would use magnets to hold the lid firmly closed until the arm comes out. (svg here)
18th Dec 2011:
Now available for sale, a little bit smaller than this one but functionally and aesthetically the same.
This kit can be purchased through my shop here