It doesn't look very pretty when I'm doing the initial hacking with a No.11 Xacto blade. It all starts by drawing out a rough area that I want to carve on the surface of the plastic, and planning so that I don't cut away internal detail or structure that is necessary to hold things together. Then I start to "drill away" at the corner points of those areas by simply twisting the blade into the side of the fuselage like a drill bit until I have a hole 6 to 10 mm in diameter in a few places. Then I start the more gentle carving with the same blade to join the holes into one larger area. The last part is the finesse work with the same blade and a file and a sanding stick to smooth the curves. At this point I'm eye-balling it - just shaping everything into gentle curves that flow into and out of one another. At the same time I try to thin the edge of the hole by beveling the interior surface edge so that one doesn't see the huge scale thickness of the plastic on the cut edge.
One drawback to this method is that once the cuts are made - the cut plastic is just a bunch of shavings. The method you used for cutting away the Shackelton is something that I have not tried . . . yet.