[aerohead]

I second the motion on following MetroMPG's cardboard and duct tape rapid prototyping.
A receiver hitch cargo carrier,or lashed-on bicycle carrier can support a mockup of a tail for testing.
A weekend of fabrication can get you something tough enough to withstand road testing.If you like the results,then you can slow down,and invest the time and resources to fabricate something you'll enjoy,without wondering if it will all be for naught.
Since the performance of a boat tail is predicated upon quality airflow coming at it,you might want to consider an under-body cleanup,and full,rear wheel skirts,to help reduce any obvious turbulence upstream of the tail.
Canoe fairings for the rear wheels would also help for 3-seasons driving.