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A flat underside offers the best drag, but it's not practical, especially when you don't have the freedom to move the rear wheels (inwards and rearwards would be ideal). For best aero, you probably want to go with the shallowest departure angle you can get away with.
You will want a curved shape. Airflow may not be able to make the transition around a corner, which would result in high drag. It's gotta be smooth. But if you want to test a hybrid design, you could build it in cardboard and tuft test it to see if flow remains attached.
As for materials, whatever gets the job done is fine. As long as it's got the right shape, there's no aerodynamic difference between materials.
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