The quick and dirty....

#1 make a copy of your fuselage

#2 Invert all polygons

#3 Scale slightly (call it .98, .95 in x, y (leave z=1)

#4 at each opening, make  a 1-section tube (your "frame") with the same number of points per section as your window (opening) outline.

#5 Snap the points on the "outside" section to the "outside" opening points,

#6 Snap the points on the "inside" section to the "inside" opening points,

#7 - *MAKE SURE* the polygons on your "frame" are pointing inwards.

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This is one way .... and best done after you've finished your "outside"