The K&N Short Ram Intake is a common modification that I see on the 02-07 WRX and 04-07 STI. It comes with a heat shield to help keep the heated engine bay air from entering the filter. The rubber edging on the heat shield (marked by brown dots in the picture below) seals against the hood, to form a box, of sorts.
Fresh air for the filter has to come from the stock plastic “funnel” by the grill area, through the resonator, which has a 180* bend in it. To shorten the path and reduce resistance to the air flow, a “U” shaped slot is cut into the side of the funnel. The resulting “flap” is bent and held in place with a zip-tie, closing off the path to the resonator and at the same time redirects air toward the filter.
The modification is shown for the K&N intake system but is applicable for other brands as well. In addition to being a simple and free modification, it is reversible; just cut the zip-tie and move the flap back.