Okay.
Tried again following Tig's instructions more carefully. This is where I am now. Do i simply make a copy of the curved group and then connect the two via lines, or...? Tried that but it doesn't seem to "connect" the two curves.
Edit: Didn't quite understand this at first bt since my curved surface isn't the same width at top and bottom, the top and bottom isn't the same "depth" either. Hmmm... Sure glad I wanted to do this in Sketchup before I started building anything!