Hi Pilou, there is not currently any very good method to make a result like that directly in MoI - using Shark or Rhino to do it and then importing the result into MoI would be what I would recommend currently.

That's often times called an N-sided patch, and it's an area that the people who make the geometry library that MoI uses are actively working on right now, so I hope to add that functionality in to MoI but it's not in there yet.

If you were to extract that surface all by itself and turn on its control points you would see that the actual surface extends larger than the area that you see there, it's a bigger surface that is fit to pass through those curves which then has trims applied to it to cut the outer area away.

If you were to try to construct it manually in MoI without using any n-sided patch command, you would probably try to do a similar thing of building an initially bigger surface that would then get trimmed. That can be tricky though.

- Michael