Since I haven't had much time to work on the tail, and with a 1000-mile trip to Connecticut this weekend (where I'm writing this, in view of Rhode Island), I decided to fabricate a temporary tail on a whim Friday night out of coroplast:
The trip today netted 61.0 mpg on the display, at 70 mph on cruise control. Not bad for something thrown together at the last minute, and with all those sharp angles.