Prius guys (or gals) stick foam pipe insulation between their grille slats.

On my Camry Hybrid, the foam pipe wouldn't work. So I bought some foam-core "poster" board and zip-tied it to the inside of my grille. Poke 2 holes through the board where the grille slat it, put a tie through the holes and around the slat. Works very well. And is nearly invisible.

A side advantage I see with grille blocking, is less salt spray inside the engine compartment!