Why does it need to look pretty?
Its an engine bay, with an engine and associated parts inside, and is completely enclosed by the bonnet, wings and front grille, which if your really lucky are already painted and polished and match the rest of the car.
Do you polish the underseal underneath the car as well?
Seems completely retarded to me.
If you want to show off the engine bay, then show off the engine bay. But covering the whole engine bay in plastic just defeats the point. The bonnet does a better job of hiding the stuff, so leave it closed.