Looks like a pretty good design, I like the light placement. I had a headache rack installed recently (the link above) and the visibility through the mesh is much better than I expected, even with the line-ex on the mesh, so I'm not sure you need the unmeshed middle section in your design (if it's purpose is to increase visibility). It you're going for functionality, full mesh coverage would be better IMO. A full mesh coverage that was removable would really be the ticket for versatility. Toward the bottom of this page is a pic of clamp on mesh
If your mesh ends up being permanent, make sure their is room to get a squeegee in for cleaning the rear window.
Also from a functionality standpoint, I think some sort of bracing to increase rigidity would be helpful. Sounds like a fun project, keep us updated.