6" machinest rule, the rigid kind, contact cement and sandpaper and you can finish the tone board to any finish you want and it will be flat. I usually don't use the cement. I just wrap the sandpaper around the rule a couple of times and then just change sides as the paper fills up.
You might as well buy several. The edges are square and they make great scrapers for the tone board before you sand and the edge is straight so you can use it to see if the tone board is both flat and level.