[aerohead]

Since the Beetle is kinda tall with respect to it's width,then consider using a symmetrical wing section as your 'Template' in plan view.
*On the last page of the 'Aerodynamic Streamlining Template Part-C' thread is some data on sections.
*The second pictorial drag table has the lowest drag section.It has a chord-to-thickness ratio of about 3.9:1.You can measure it off the table.
*Anything 'thinner' or more 'bluff' will have higher drag.It is the drag MINIMUM.
If you'll match your roof curvature to the sides you should have zero longitudinal vortices.
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If you're going to do rear wheel skirts,then I recommend you leave their width alone,but add fairings in front of,and behind them to soften the pressure spike which they cause which will impact the boat tail.
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If you're going to have exposed wheels and run MOONs,then I recommend you reduce their frontal area by trimming them and extending them down close,and flush with the outside of the wheels.
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If your tail extends only as far as the rear edge of the rear bumper you should see Cd 0.244 territory as with the 1934-1/2 Chrysler DeSoto Airflow experimental.
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The inflatable tail will get you lower.
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*The Rumpler Tropfenwagen only achieved Cd 0.28.You can do better.
*The Dymaxion Car is estimated at Cd 0.25 by one source.You can do better.