Use this for the kammback; you'll get more accurate measurements
For the grille block and stuff, I'd buy some thin acrylic or other bendable plastic and tape it onto the bumper from the painted lower lip all the way up to the matte black bumper bar. That way it will be exceedingly smooth and the air won't hit the little flanges on the edge of the grille. Start from the sides and do the center last so the air doesn't get trapped.
If I'm not being clear let me know and I can Photoshop it!