How about a stretched canvas instead of plastic?
We are not aiming for extremely high speed: 50 mph is more than enough with a car like this. That speed might not demand a robust metal bonnet. Stretched canvas is widely used in boating; how about using it on a car? It is very lightweight, provides a car-like outfit and helps to reduce air resistance.
Did you notice that there are no doors? Actually, minimalism is addictive. The more we think about minimalism, the less thing we believe is a must. It is fancy to jump in, isn't it? Elderly folks might struggle to step over the frame, so we might need a version with a door if there is a demand for that.