Have you tried a small opening in the center of the lower grill? Having the upper grill completely covered is better because this moves more air up and over the car. If the opening is at or just below the stagnant point, the air flow into the opening is better, so you can get by with a smaller opening, than if it is less optimally located.
Does the "chin" area have holes in it? If it does, then blocking them will force more air through the radiator, which then lets the lower grill opening to be smaller than it otherwise could be.
Here's my current grill block on the Scion xA, which is virtually the same car as yours:
(click on image for link)