The pic I google searched the image 06 Matrix. I use MS Word and I use the Draw Feature to render the lines. I save it as a Pdf, then save it again as a JPG in Acrobat.
Again, Toyota did their homework to make this a pretty aerodynamic car. The air coming off the roof and sides has been optimized for this length and height. If you add only 1 foot or less to it, the chances of you getting it right are pretty darn slim, at best you could equal the Cd, but it's a 80% chance you'd make it worse. If you go longer and follow the "Template", you stand a better chance of making a shape which will help you simply due to the length. A longer extension will make the final cross section when the air leaves the extension smaller.
And the Green line is my idea for a boat tail.....I can't begin to say it's an "ideal" shape as many in this forum will answer with a hearty "AMEN"!
The Black line is the min I'd think you could productively do, but that's just me.
