[lunarhighway]

on most cars the top grill is more for looks than aero and generally in a less high pressure area. on a large truck is is probably less so. open the hood and see where the actual radiator is... also see where the engine breaths air. sometimes you can get away with blocking the whole grill be FE and performance falls as the engine breaths hot air trapt in the engine bay. initially make something you can quickly take off and take the tools along to do so... if temps go up to high you can correct the situation.
carboard is great for initial testing if it's not to rainy... it's more available than coroplast and if your first attempt is not successfull just throw it out