[aerohead]

For those nuts enough to desire elongating their vehicle, the following is a very 'old-school' technique for quantifying their vehicle's OEM wake area, in anticipation of creating the 'scaled-down' wake footprints, needed to form the new tail's bulkheads.
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Background:
Before CAD and AUTOCAD, complex-shaped areas could be found by accurately weighing a 'photograph' of a shape, and comparing that weight with weight of a reference area of the same photographic material.
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Cardboard is manufactured with remarkably 'tight' quality-control standards, and it's unit density ( mass/weight ), per unit area ( square-foot/square-meter ) is remarkably uniform as well, as with photographic prints.
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The idea is to, create a cardboard pattern of your car's wake, by transferring dimensions directly off the car and onto carboard.
1) An accurate, perfectly-square 'rectangle' of cardboard, equal to the net width and height of the wake is first cut.
2) Then you need to find something like a high-school, or junior-college laboratory, which has a scientific-quality electronic, load-cell type scale, with an accuracy in fractions of a gram, and weigh your cardboard rectangle.
3) This is absolutely critical to the project!
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4) Then, transfer your 'shape' onto the cardboard and cut off the margins as accurately as you possibly can ( you're leaving only the 'shadow' of the wake ).
5) Now, take your pattern back to the 'lab' and re-weigh it.
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6) You knew the original area of the uncut rectangle, and you knew it's weight.
7) Divide the weight of the 'cut' cardboard, by the weight of the 'whole' cardboard.
8) This is the 'fraction' of the original rectangular area remaining.
9) Multiply the original area of the rectangle by this fraction to arrive at your OEM wake area.
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If we accept the linear relationship between wake area and drag, then any desired Cd can be computed simply as a function of reduced wake area.
If you want to cut your Cd in half, reduce the wake area by 50% ( assuming that your contour is streamlined enough to maintain attached flow all the way to the new separation line, top, sides, and bottom ).
( anyone with 'Aerodynamik des Kraftfahrzeugs' will see the genesis of this relationship through the researches of Walter Lay, Baron Reinhard-von Fachsenfeld, and Dr. Wunibald I. Kamm ).