Rather then adding a boat tail, chop the roof / reshape the hatch to become the boat tail. My beetle roof is ridiculously high, a easy 12" above my head, i know a basket ball player could sit in it, with his knee's up by his ears, but his head would have plenty of room. By cutting you would be reducing and reshaping the mass.
Something like the old karmann ghia's only more matched more to the template.
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Ride a Mtn bike for errands exercise entertainment and outright fun
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