I did this to my car and went with an oval shape. I got rid of the vertical hood latch piece that goes to the lower frame rail and put a horizontal piece that crosses the radiator. You can't see it because it's behind the bumper.
I thought about having an opening where the license plate fits on a street car, and a solid plate would completely cover it, but a fine mesh could allow air to pass through while protecting the radiator and could even be subtly painted or decorated like a plate. Wouldn't be quite as large as an R apron but it would certainly cool a trans or oil or power steering cooler.