Hi, I want to make a curved doom-like shape on top of the flat surface A. Is Boundary Surface the best way to do it? When I tried to  use Boundary Surface, Preview showed that the region above (1) is not included. I guess I need to somehow join line segment 1 with line segment 2 at the junction indicated in blue first. Am I correct? How to do that?

When 3D print it, will the surface be printed as a thin sheet on top of the flat surface A or the empty space between the doom-like shaped surface and the flat surface A be filled? I would like the space be filled like a solid object. Thank you.