Home made fan shroud.

A lot depends on when your the scoop is on the hood and the size and shape of the scoop. It may be in a high pressure zone, pushing more air into the engine compartment at speed. It may be in a low pressure zone, actually pulling air out from under the hood.
An air dam under your bumper will block air from going under the car, thereby creating a low pressure zone underneath, sucking more hot air from the engine compartment.