Just in case the Sharpie leaves a mark (and it might) I had a thought. I have not done this but thought it might be a good idea. How about taking the board cover off and folding it in half lengthwise (to create a crease as a guide) and then sewing a straight line with your sewing machine following the crease. If this works (again I don't know if it will) then you could add different angles as well, such as 45, 30, 60 and anything else you might need.
It seems like a good idea in my head but who knows if it will be practical. Hope this helps. Just a thought.