I had another idea about an aero-cap...
In the mind-set of "do what you know", I wonder about an aero-cap based on the design of a "teardrop trailer".
If you haven't seen one before, here's a good example.
They tend to be not as aerodynamic as they could be (a little too round, not pointy enough on the back...) but a cap based on the general design of a teardrop might work well.
Aluminum sheet is weather proof and durable, and looks pretty nice.
I built a teardrop trailer a few years back, so I already have the experience of working with wood and aluminum for something like this.
Just an idea... just throwing it out there.