Hi Finema, the seam is going to end up at the start/end point seam of the closed curves. Right now there isn't a tool for changing that directly but there is a kind of roundabout way. The roundabout way is to do a Loft just between your 2 closed top/bottom curves. While you are in Loft it is then possible to drag the seam points that are shown in the viewport to change the seam of the loft surface. After you have moved those to the back and finished the loft command you can delete your original curves, extract the Loft's edges using Ctrl+C/Ctrl+V and use those extracted curves for the Network.

- Michael