I'd like to offer some advice. Reading this report
http://csus-dspace.calstate.edu/xmlu...pdf?sequence=1 gave me some information I think you might find useful. It's about testing pick up trucks and aerodynamic bed configurations. These are the drag coefficient figures for aerodynamic toppers simply cut at an angle downwards, instead of a suburban like back end or a pick up truck bed.
Baseline 0.3453
Traditional canopy 0.3157
5degrees 0.2957
10degrees 0.2894
12degrees 0.2892
15degrees 0.2987
18.7degrees 0.3091
Now as you can tell the 12 degree slope has the lowest drag coefficient, however it isn't much lower than at 10 degrees or 5 degrees.
This suggests to me that in this case, instead of following the template as you would do on a boat tail, you may want just a slight taper on the top and sides. Even just a few degrees. That way you end up with a fairly significant drag reduction, without impeding the usefulness and spaciousness of the topper.
If it isn't going to be a huge engineering challenge I think the pop up design would be very useful.
Whatever design you do choose I'm looking forward to seeing it all come together.