TheExtrusion process automatically adds patches. I had 'practised' adding faces, as indicated earlier. The back open ends would have to be 'patched' too, to make Solid objects. But I will do this after extruding the back Outline, as this is only the 'Bevel' Stage. This looks like an extrusion but it is an exaggerated Bevel.
Faces look 'mushy' in rendering.
After Bevel, Then Regular Extrusion can take place. Select backmost splines, choose extrude and send back in the Z axis. Don't forget the Counters. This Side view shows that Faces are not flat. There is a mistake.