About a curved thermal boundary

Hi, I am currently try to use the PLAXIS Thermal 2D to build a thermal boundary for the tunnel interface.

However, it seems like the Polycurve inside PLAXIS is not able to create a thermal boundary for it. 

Do you know if there is any other option that I could use?  If I just use the linear thermal boundary to represent the curved boundary as the figure below. It is not quite accuracy, right?

Looking forward to your kindly reply.  Thanks a million.