For my 4th/5th grades I am doing a manhole cover project that I hope will be done in time for open house. I'm planning on using 2 pizza rounds hinged together. The top should look like a manhole cover, and underneath the students will make up their own vision of "what's down there"... underground city, outer space, ocean, pipes... just about anything. I plan to put these hinged pieces either lined up on the floor in the school hallways or on the cafeteria tables. I'd like some ideas on how to make the manhole cover parts look the part. I'd been thinking that some kind of raised line to make them 3D would be great, but can't figure out how to do it, esp. to keep the cardboard from warping too much. Any ideas out there?