Block the top underneath so you don't see it AND block out the bottom like .....
You want as much air as possible going around the sides of your car, and over the top. Even if your grill block is behind the upper grill, it'll still be very effective because the air won't be smooshing its way through your engine bay and forced to speed up. Blocking on the sides of the lower will force more air to go beside the car. Hope this helps.