Leaving the tail lights visible could be problematic. I'd rather do the "box" on the rear end, which extended could look like a four-sided Bonneville spoiler.
I wonder how much more benefit you get for lengthening it (within the template of course). If I could make it 20mm or 30mm long instead of the paper's 10mm perhaps it could show a 5% improvement...? That would be about 1-2mpg. But even then I'm not sure it would be worth the time.