In the Object Library you can find objects to construct tapers with vertical walls (Isosceles Trapezoid) and also for creating a straight waveguide with angled sidewalls (Straight Waveguide). For the “combination” of both structures, a taper with angled sidewalls, you can use define a planar solid structure inside a structure group, as in the attached example file taper_angled_sidewall.fsp (236.6 KB).
In order to define the planar solid structure it is necessary to specify the position of the vertices as shown below. The order in which the vertices are listed defines the labels when defining the sets of points that form each of the facets.
The labels of the vertices are shown in the image below, together with a front view of the taper. The parameters used to define the geometry of the taper are also shown.