I remember working on that. It was tough.
See my attached model.
1. Make the curve in Sketch3 not just tangent, but also curvature continuous.
2. Set the end conditions of each edge to not be tangent. I don't think they are on iMacs. I softened them back a bit.
3. First create reference surfaces, and use those instead of lines.
4. Add a center surface using boundary, then trim it back.
5. Use a curvature continuous face fillet in the corner (Fillet1).
6. Finish off with a diagonal control curve.
7. Add two connectors (boundary surface 12).
rearcomp_cc_2012-08-21.SLDPRT.zip 617.2 KB
This worked out great!