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Measure your grille's widest dimension, and it's tallest dimension.

Get some 1mm or 3mm lexan sheet from Lowe's or Home Depot or somewhere similar.

Get some 3m trim tape (double sided adhesive used for automotive badging.)

Cut strips of it that are as wide as the outer most lines of your upper grille.

Lay the lexan against your grille, mark the outline of the grille, trim accordingly.

Finish trimming and fitting up the lexan without adhering it.

Once you're satisfied with the fit, you can either paint the piece, or leave it clear (OEM appearance).

Apply the pieces of tape that you've cut to the grille.

Place the new grille block over the tape, pressing firmly at every place where it makes contact with the tape.

If you ever decide to remove it, it can be done with a sharp razor blade and tape-removal cleaners.