"Showerboard" ghosts excessively and is very hard to clean. I ended up with a big sheet of clear acrylic from "home depot" instead. It was 36" x 72" (3 feet by 6 feet) and cost about $48. It worked perfectly, was easy to hang because its quite rigid and makes a very cool effect when you shine a light on the edges. (The marker marks light up a little).

You can find the stuff in sheets of many sizes by the window and glass repair stuff.

Waxing the showerboard helps. I'd suggest experimenting with different brands of automotive wax to see which one is easiest to clean without resisting the markers. My middle school geometry teacher used waxed showerboard (over 10 years ago) and it worked quite well. Thanks for the idea of using the acrylic. I may have to try that; maybe I can combine it with a FTIR multitouch system or something.

Thanks Jon. I've never tried making one myself. Took us 6 months to make a mass-market version of this to help out startups.