I've been looking into doing hood vents on our cars for quite sometime now. However I haven't found something that will really work well. Problem is our hoods have a nice contour to them so it makes flush mounting and kind of vent/louver difficult to say the least. I actually have a set of those Grand Prix louvers that I plan to install on the custom hood I made for my Camaro, but that will be easy as I have a flat surface to work with.
As far as the Contour goes... the best hood louver I have been able to find that would work (I think) is the Cosworth Sierra hood louvers. They have the same style look as the Grand Prix louvers do, but they are much shorter in length and I believe could be made to mount nicely on our hoods. However they are expensive as the true Cosworth louvers are aluminum and out of production. That said I have found some UK and Australian companies that make fiberglass replicas for around $50-70 shipped to the US. Still a pretty penny considering I bought the grand prix louvers for $30. However the $50-70 beats the ~$229 for the replica aluminum version. Here's a picture of the louvers I'm referring too...
And a quick photoshop of what they could look like...
EDIT: just FYI there are water catchers that came factory with the grand prix louvers that bolt into the bottom of them. However they are discontinued from GM and are fairly tough to find... but definitely would look better then that glued on box that's shown in the pictures you posted.