I built an interactive whiteboard as a school project. The pen design I used is pretty standard, but I cut out barrel of an old Expo whiteboard marker to fit everything inside. For the momentary switch, I ripped out the reset button on an old computer we had in storage.



It cost around $2 for the IR LED and AA battery holder. The other parts I found laying around.

Here's the project page for it if you're interested: http://make.greshamhs.org/?p=4