At least three members here have expressed an interest in building a boat tail for their 2000 Honda Insights, so here are a few of my notes on what such a boat tail would look like.
I started with the side-view image from the InsightCentral encyclopedia, and checked the existing angles of the car's rear end. It's about 8° in the vicinity of the antenna, 12° around the middle of the hatch glass, 14° for the rear third of the glass, and it could plausibly be 18° at the spoiler.
Here's a picture where I extended the roof at 18° and the bottom of the rear bumper at 7°, and truncated where it is 15.5cm tall for the license plate. This results in a lengthening of 1.48m, making the car 5.42m long. For comparison, a 2010 Accord is 4.93m long, and a F150 short cab short bed is 5.41m long.
The 18° and 7° angles are from EM conventional wisdom, which may be from Hucho's book. However, I won't believe it for certain until I see it in a wind tunnel or on a prototype. Particularly, what's the relationship between the angle of the bottom of the tail and Cd?
The design of the top is easy, and the bottom is fairly straightforward, too. But what will the sides look like? How far forward will the tail extension need to start? The rear is rounded, and it's not clear exactly where flow becomes detached.
Btw, the rear of the tail will be about .8m wide, which is almost large enough for stock tail light housings and a license plate, but I was really hoping to leave the stock tail lights where they are.
My next step is to create a 3D model of the Insight, add a few different styles of boat tail, and see what happens in the SolidWorks FloWorks simulation.
I found a program to convert a .JPG into a .DXF that SolidWorks can import as a sketch, and I made .DXF's of the top, side, and front views. I'll combine them into a model some time soon, and I'll make the files available if anyone is interested.