rj, I have my own grille opening completely covered. See it in my garage.
You have a nice flat surround on the outside of your grille to tape to. The coroplast should work for you. When you decide how much grille opening you need, I still would cover the entire grille then cut a crosswise slot an inch or so wide to line up with the centers of your fans. With a narrow slot, it is easy to add to or subtract with tape when needed. After my grille block was duct taped on I masked around the tape real tight and painted the tape and the block at the same time. Looks fairly decent.
Don't wreck your paint job.
We all gotta start somewhere.
Good Luck. CO ZX2
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