Good job, SCNTN!!! So you're leaving it open for the GMC logo to show? What a great idea!
When I was forced to cut the 6" diameter hole in my cardboard grille block (which got rained on, ruined, & is now long-gone), it exposed PART of the GMC logo. I thought, "yuck - that doesn't look too good" (as though solid cardboard does, eh?).
But that's also part of what's making me think of doing it out of clear plastic. Right now I have a couple smaller pieces of clear plastic there - a "partial" grille block - and in most cases, it's almost "invisible" (at first glance, at least). So if the whole thing was clear plastic, I'm thinking it will be very "low profile" looking. Not as much as a true "stealth" block behind the grille, but far better aero .... the "aero stealth" design. HAH!!