I made my own out of fiberglass. Here's an unfinished picture:
And one mocked up under the hood:
I'm curious about the aluminum one above. Does it have some contour or is it more of a "box?" I liked the idea of an aluminum shroud but didn't have the provisions to make a nice, contoured shape. The good thing about a flatter aluminum shroud is it's easy to make "flaps" that open when the car is moving to improve flow.