how to transform a closed Parametric curve in a 2d surface

Hello comsol forum

I am trying to simulate scenarios of 2d scattering of a cylindrical em wave from an infinitely long z-directed PEC with a specific cross section.

I successfully done this for a circular and elliptic cross sections. But i wish to do it for other cross section shapes also, for example a rounded triangle or a rounded square. From the parametric curve option i can design the cross section but i don't know how to transform this curve to a 2d surface in order to use it in the same way as the circular and the elliptic shape.

In other words, how can we transform a closed parametric curve to a 2d surface in order to use in a 2d scattering problem?

Thank you miko