- Replace the shorter section curve which is degree 5 curve with a new degree 3 curve.
- The curvature of the vertical sides is at or near zero at the quadrants so it is legit to make lines (red) and BlendCrv them (bonus blend tool) for curvature (Blue curves). The lines and the ends of the new blues curves are located at the control points of the section curves. The section curves have the same # of points.
- Loft the blue curves and the lower existing curve, which is the top edge of the vertical surface. Loft to a point at the top, and use the Loose option. I used the rebuild (10 points) option as well because the lower curve in the loft does not match the new blue blend curves. . Use MatchSrf to match the new top to the vertical sides.