I use the contour gauge, but trace the outline on a piece of cardboard, a bit to the outside of the line indicated by the gauge, by maybe 1/16. Then I cut and fit, a bit at a time, till it fits properly.
The cardboard I use is actually a piece folded in half. The fold line becomes a centerline, and I draw the indicated contour away from the fold. Then, when I get it cut out to fit exactly in the fuselage half, I assume the section is symmetric, unfold the piece and should have a full section. Usually any trimming for asymmetry is quite minor.