Lay the port (left) lower cabin side on the bench then placeC-1 into place.  Ensure that C-1 is aligned.  "Tack" C-1 in place.  I used a straight edge to align C-1. 

  Place the port (left) lower cabin side and C-1 in the deck opening.  Align the side in the opening and install the starboard (right) lower cabin side.

  After the sides are aligned in the deck opening, tack glue the starboard lower cabin side to C-1 with some CA.

NOTE: You will notice in the picture above that I installed the 1/8" sq. spruce to the inside forward surface of the right and left, lower deck sides.  This was not glued past where C-1 goes.  Because I will install C-5 later in construction, the spruce helps to hold the sides next the the decking.   The spruce will be removed before I install C-5.