Make your own. Cut it open with a circular saw after measuring and marking. Glue in fillets on each side, fill and prime and your done. Just as easy or easier than installing a pre moulded unit. That being said, there are some nice aftermarket ones out there. This is not my design, but a imitation of the Archie hood which was originally designed by Rich Warber. They really do allow the exit of a lot of warm air and air pressure from the radiator area. Hope you find one or make one because they are not just a cosmetic add on!
The screen for the vent is very sophisticated stuff! They are a gutter guard screen, and they are available at Home Depot or your local building materials supplier for around 2-3$ a piece. About 3 feet long and 5" wide. I cut them as needed for my side scoop and hood scoop projects. It is an expanded aluminum and looks pretty good for the price.