Put a sheet of paper or cover stock where your hand rests?
Actually, I think an app-based version of this would be the quickest solution. I don't have the devkit but wouldn't it be possible to implement a square/rectangle shaped space in the app which would ignore input? You could make the shape adjustable and movable so that you can comfortable rest your hand on the iPad and the only input it would accept would be from the portion of the iPad not covered by this shape.