Then a way to form your inset panel is to select your main shape and run Construct > Offset on it with the "flip" option set to make a shrunken down version of it that is contained inside the center of the original.

Then draw in additional profile curves in the side view:





Select both the body pieces (the main one, and the shrunken down interior one created by offset), and run Edit > Trim and use those curve as the cutting objects.

Then delete the inner portion of the outside body:




And delete the outside portion of the inner area:





Then select pairs of edges and run Construct > Loft to make connecting surfaces.

If you want the connecting pieces to run at some other angle then you may want to trim each piece to slightly different outlines, you would probably use Construct > Offset on the cutting curves to make an offset curve and then use one set for trimming the outside body and the other offset curves for trimming the interior piece.

Hope this helps!

- Michael