The cabin basic design, still needs to be rounded on the spiky side edge and smoothed on the top, before proceeding to the fine filing. It is easier to re-build the shape of only the upper part of the cabin inside FreeCAD: with just one simple “fillet” action on the side edge, the basic rounded shape is there.
Smoothing the ridge on the top is a little bit more tricky: applying a fillet to any short edge doesn’t work well, as the segments composing the ridge are oriented in different angles. Then the solution is to cut away the spiky part of the ridge, leaving a smoother surface instead.
I created a profile which was cutting out the ridge, but at the same time including the whole upper cabin. The solid generated revolving it around the longitudinal axis includes the cabin, cutting out just the thin part of the ridge to be removed. The boolean difference of the two shapes does the rest of the job: the result is the original cabin, with a smooth surface instead of the ridge.
This shape is now ready to have the windows cut out, before being exported in stl format to be finely adjusted inside Blender.