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.
Step 2: Open it up
Now, carefully pull out the part that holds the ink and the marker tip, so as not to make a mess.
Step 3: Cut and glue
Next, you need to glue the eraser to it, I used super glue but I don't think it really matters. If you're using something else besides a piece of eraser, like a piece of cloth, then you can use a square of cardboard with the cloth on top of that.