[JustROLLIN]

Yes, you can add a filter right on the valve cover, but it can make a mess over time. Sometimes oil will penetrate the filter and spray into your engine bay. That why I use an in-line fuel filter and still run the line back to the intake. But, maybe people who run just the filter will have a different opinion.

And yes, you need to plug the open nipple. Air is entering your engine through that nipple and is not being metered by the MAS. In turn, your car may be running rich. Make sure you plug that open nipple up.