You can set the slice thickness for Scissors tool (set thickness to 0 to limit it to the current slice). You can also use the Draw tool (if you draw another segment then it will be erased from the current segment). You can use masking settings to limit editing inside a selected segment (so that you only need to pay attention when you draw the contour inside the segment).
You may save time by segmenting manually on every 5th slice and then fill between using the “Fill between slices” effect.