The basic lower valance is too prominent in the upper curved profile and too spiky in the lower part: a round cutout of the shape is needed. The right curve is created with an eight of an ellipsoid, placed and shaped with a fine tuning to match exactly the needed curve as well as the remaining parts of the basic shape.
The front part of the valance is then created extruding the side face of the ellipsoid, while the back side part of the basic valance is kept. With a little work of extrusion and boolean operations, the final lower valance full shape is there.
For the final cabin, an empty shape is needed, with a given thickness. It’s enough to clone (in green in the picture) the full shape, place it backwards and inwards of a distance equal to the needed thickness and cut it out from the original full shape.
Now the new top face of the lower valance must be extracted and used to make the upper cabin profile match the lower valance.