it makes a BIG difference..
not only will it force most of the air though the radiator instead of everywhere, it will so decrease the dead spot in the center where the fan motor is and won't be as noise from the backpressure from the radiator.
its not a slight improvement, people, as well as myself, have reported a large drop in temps.
if u can't afford a metal one from a store, you can look for a tupperware thats about that size and cut out holes to your fan/shroud. or u can do what i did and use cardboard.