Along the lines of what Basjoos mentioned, but without actually extending the car (which most consumers aren't willing to do), how about a replacement cargo-expanding rear hatch with Kammback dimensions:
Aftermarket rear hatch with roof extension
Make rear hatches for the most popular models, make them a simple drop in replacement, and add to that a hitch-mounted cargobox tailored to the rear hatch and you've got the better part of a boattail.
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e·co·mod·ding: the art of turning vehicles into what they should be
What matters is
where you're going, not
how fast.
"... we humans tend to screw up everything that's good enough as it is...or everything that we're attracted to, we love to go and defile it." - Chris Cornell
[Old] Piwoslaw's Peugeot 307sw modding thread