The fuselage structure is made using repainted versions of the kit pieces glued to 0.02" thick cardboard. I have had good success with this thickness cardboard as it matches pretty closely with the thickness consistent with the thickness printed on most kit parts. I source my cardboard from promotional materials I acquired General Electric Power System Promotional materials. Cheap is good. The cardboard is sandwiched between layers of a slick paper material, which perhaps adds to its good properties, although I have to rough up the gluing surfaces to ensure good adhesion.
The structure is in two parts, behind the instrument panel and at the instrument and forward.
The cockpit is repainted in parts but structurally is the same as the kit design. I did not do any research so to make modifications to make the cockpit more accurate. I deviated from the kit assembly by attaching the instrument panel to the front structure. The kit design has the instrument panel attached at the right and left ends into notches cut into the two sides of the fusulage structure. I thought this to be two weak and I also wanted to be able to line up the instrument panel top surface with the rest of the structure so the interface to the fuselage covering along the width of the fuselage would be smooth. I smooth out the structure using sanding sticks so strong is good.