Thanks for posting that David.
Good idea you've had there, I know you mention in the blog that it is quite difficult to hold the jig against the face, edge and mark the pencil but I wonder how else you might get around the problem without spending a lot of time developing something - perhaps like a width guage that runs up both edges as was suggested earlier in the post.
Short of rigging up something with adjustable legs and wheels, like a clamp that can be secured tightly to the edges, or some sort of overhead frame that you place the bat under and secure and use the frame to guide the marking implement I'm not sure how else you can easily do this.