aerodynamic

I have a 95 Acura Integra which I have fully converted to EV. My question applies to just about any front engine (motor) car. What material can I buy and install that will close off the bottom of the engine compartment? It would seem that some form of plastic (or maybe aluminum) about 1/8"...

http://www.evconvert.com/tools/evcalc/ Simply amazing. However, there is no preset for a 400 lb motorcycle, so I have been tinkering with the numbers under "1988 Honda Civic DX Hatchback" - basically wildly guessing. My bike is a '78 Honda CB400T. No fairings or windshield. Does anyone know...