found a good pic of a nice FOR-ECOMODDERS radiator duct

[IsaacCarlson]

It would be easy to build one and I don't think it absolutely has to have rounded sides. The rounded sides just help the air expand more smoothly and help the air build pressure so it doesn't just blow through one spot on rad. If I can get a hold of some coroplast, I will try it. I know it will help a lot in my case because most of the air goes around the radiator anyway. I did manage to get some small-holed hardware cloth over my grille opening to keep rocks and bugs out. They were doing a number on my radiator after my condenser removal.

[Daox]

Nice picture Isaac.

The Toyota Prius already does this as I posted a while back in my Prius build thread. I also did some filling in on the Paseo as well to try to improve airflow through the grill, but to a much lesser degree than the Prius has on it.

I'd definitely say it could be used to make the grill opening smaller which would further reduce drag without compromising your cooling ability.

[IsaacCarlson]

very little air that enters the grille actually goes through the rad. It is completely open around the rad!!!! anybody else have this issue?
It is almost as if the radiator was an after-thought

[daqcivic]

I've actually done this with coroplast on my car, in conjunction with a grill block and undertray, and even in the summer in stop-and-go traffic temps never got that high--not even tripping the fan for longer than 15 secs at a time. Of course, aero doesn't matter in such conditions, but it illustrates the basic effectiveness of the design. I would venture to guess that I can now make the grill opening 2/3 the size of what it would otherwise have to be.