I was hoping to get the aerodynamics of the tail nailed down in SolidWorks COSMOS FloWorks, which can illustrate streamlines but can't tell you overall drag. I plan to run a few variations on a model of the car to check for flow separation in parallel flow and in gentle crosswinds, then build one a little bit more conservative than what the simulation tells me I can get away with. Here's an old model that's similar to an Insight:
I could make a much better model by importing the side, top, and frontal views of the car, then adding detail where necessary.
For construction, I really have no idea what I'm doing. Maybe an Al frame with the bottom half of the skin coroplast, and the top half acrylic. I'm sure you'll be going with foam and fiberglass. Will that be on a wooden frame?