[aerohead]

I don't have the New Beetle with me,but I've got the New,New Beetle here.
The 2011 Beetle has a Length/Height ratio of 2.878 and Cd 0.37?
*1999 Prius-I ------------------------------2.876 and Cd 0.29
*1991 GM HX3 -----------------------------2.856 and Cd 0.258
*2000 Honda Insight-I --------------------- 2.854 and Cd 0.25
*1980 VW 2000 --------------------------- 2.781 and Cd 0.25
*1987 Renault Vesta II ---------------------2.772 and Cd 0.186
*2008 M-B Boxfish ------------------------- 2.560 and Cd 0.19
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All these cars have 'shorter' fineness ratios than the Beetle,but better rooflines and sides.
Extending what you have along the AST contour as far as you dare will pay the highest dividend.If you went a little further,you could correct the rear fenders as well.
Going as far as the rear bumper,with a pure vertical chop (like Carl Breer did with his 1934 DeSoto Airflow (the world's 1st actual 'Kammback)) would yield the lowest drag with zero added length.
And it can all be open at the back,with 'steps' inside,like Ford did with the Merkur XR4Ti bi-wing spoiler.The flying-buttresses will create a perfect hybrid box-cavity/boat tail,forcing where turbulence can or cannot be.