I thought maybe a third arm would work but I was too lazy for that. I made this instead. :-P
This really isn't all that high tech or difficult, but I made it, so I'm going to share it.
Step 1: Materials
-an old whiteboard marker that doesn't work
-something soft to use as the eraser, I just used a piece of an old eraser, but a piece of cloth would work too.
-a saw, or something to cut with.
It should take about 5 minutes to make this, plus drying time.