If you know that different wave ports on the same plane do not actually interact in that plane, you may use custom extents to divide a complex port definition into several simpler port definitions. This is demonstrated in the following image, where we've modified the previous geometry by defining a second port with custom extents.
Now, the eigenmode solutions for the two ports will be calculated independently. Instead of extracting four modes in one eigensolve and attempting to combine those modes appropriately, the solver will perform two eigensolves, where the eigensolve for port 1 will extract three modes, and the eigensolve for port 2 will extract one mode. The benefit is that the 4th order mode will never be extracted or required. While this geometry is very simple, this is a powerful technique in complex systems with many ports and conductors on a plane.