When the corners of the lid rest exactly in the ends of the two
grooves it will exactly fit the full length.
I use an old moving fillister to start the two marginal fins
that will slide in the grooves.
I set the spur rather pronounced on the fillister to score
the wood and define the fin.
I cut each fin about an eighth of an inch below the level of the
I then switch to a shoulder plane and further define the fins.