valve cover openings

I cut those knockouts all the time as part of my custom / resto work. My favorite way is with a hole saw in the drill press. I've got a sweet Irwin 2 1/4" reserved just for aluminum valve covers. Protect any nice existing finish from your drill press table as necessary in case it gets away from you. Cut from the inside out.

Great advice on holding off on any cutting until you have the proper grommets in hand! :-D The ones that will fit aren't usually available in the Help section at the parts store either.

You didn't mention which supplier made the ones you bought diymirage, but most are fairly thick (some nearly a half inch). The thick grommets to fit the MPs et al. are getting tough to find but I've had good luck with Jeg's lately. They seem to have more in stock than Summit. I personally have had trouble locating suitable baffles for any aftermarket valve covers and have resorted to making them out of sheet metal as the need arises.

Post up some pics of your score when you get a chance! I LOVE valve covers. :-D