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.
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We all gotta start somewhere.
Good Luck. CO ZX2
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