I decided to try a new approach, using Autodesk 123D to design a ScribbleBot body, and have since posted my final design on Thingiverse so that others might also benefit. It seems to work well, while still allowing the students freedom to add on to the design. I purposefully placed holes in the body, so that other pieces could be attached, like pipe cleaners, pen caps, and all sorts of small objects. For class, we used dry erase markers on a large white board table, but the students took their bots home with regular marker pens.
Next week, BristleBots!