After the glue has dried, check to ensure that the lower cabin sides rest on Frames 7 thru 10.  You can also note the gap in the deck opening between the lower cabin sides and the deck.  This gap should be taken care of when C-2 and C-3 are put into place.

  Assemble C-4 and then C-2.   Before I assembled C-2, I placed a 1/8" piece of spruce in the center when the lower cabin deck is located.  This will better support the deck.

  Next assemble C-3 just aft of Frame 9.  You want to install C-3 where it takes up the gap n the deck opening between the lower cabin sides and the deck.  You want a snug fit for the superstructure, but not a tight fit nor too loose where the superstructure can easily fall off your model.  Sight down each side of the superstructure looking from the aft.  Check to make sure each side is curved equally.  Look at this from above also.  When you are satisfied with the fit, glue C-3 in place.

  After the glue has dried, again check to ensure that the lower cabin sides rest on Frames 7 thru 10.