Do not put anything in the scoop itself. Its meant for maximum airflow to your intercooler and putting things in the way is just limiting it. If you're worried about rocks and little things just take a pair of tweezers or pliers and move the flaps back into place. as far as dirt and things it doesn't get too dirty unless you're driving through huge mud puddles or a sand storm.
2017 Hood Scoop Filter ?
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You would end up with a lot more dirt and grime build up on the intercooler and top of the engine with an oiled filter up there. Tiny oil particles would mist the intercooler fins, then any dust swirling through the engine bay would cling right to it. As it is, air entering the scoop is the cleanest source of air in the engine bay. After some spirited drives on gravel roads I would regularly find bits of gravel and mud up inside the engine bay (even on top of the engine itself), but nothing on top of the TMIC in the scoop. It all gets kicked up from underneath. A foam filter will certainly restrict air flow, as well as trap heat while sitting stopped. Heat that normally would freely escape through the scoop.
As far as flow goes, the WRX/STI (at least previous year models) has a big high pressure zone along the top of the hood. Enough air flows through the scoop and down the transmission tunnel that blocking the scoop off entirely results in a measurable increase in cooling from the nose of the car (ie. FMIC). Most professional Subaru race cars actually do run a reverse scoop, but with a v mount intercooler, and much closer to the front edge of the hood. Many of them re-purpose the factory hood scoop to pull clean air into the intake (or for an oil cooler). I haven't seen a single one that has a reversed OEM location scoop.
*A little wind tunnel food for thought..
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