I think
1. the direction of red arrow should follow the direction of current flow. In the above picture, the current is not flowing into the conductors.
2. you might want to draw a rectangle that is tilted 45 degree from the horizontal axis. cut the two corners of the 90 bend so that both of them have an edge making 135 degree with the horizontal axis. define your port on the rectangle with a current or voltage direction along the 45 degree line.
3. the port is located at a higly irregular (discontinuous) region where you would see complex wave propagation behavior. So I would be mindful that any lumped port define there might be very different from your real feeding scheme. the input impedance calculated might not be accurate.