Hi @dave,
I do not know of any plans. Thanks @gazpal for the offer of a Sketchup drawing.
Paul did an article a while back on shooting boards and there are a couple of drawings of them that you may want to have a look at.
You can see the article here: http://www.popularwoodworking.com/wp-content/uploads/2010/11/ShootingBoard2.pdf
Paul may have more detailed drawings. I will ask him.