The underbelly doubler
We now have in place the spine piece which has its flutes oriented so to provide the best rigidity across the span of the aircraft. Heavy crashes, nose first, will create large forces that will try to crumple the Evo from tail to nose. The underbelly doubler is a very useful way of vastly increasing the plane's strength in this direction.
Since the measurements in the plans for this piece are 'nominal', you'll need to put the piece into place without glue and then trim the width at the nose - to match the exo doubler - and at the tail for overall length.
When the piece is trimmed correctly, glue it to the underside of the Evo and then, if desired, tape over the join where the underbelly doubler meets the exo-doubler.