There are a lot of people on here that have used Plexiglas or black plastic and fastened it directly to the plastic grill. Lots of automobile manufacturers are now starting to incorporate grill blocks into their grills in the same way (black plastic behind the "grill"). They work almost as well as a block out front. I can't comment on your specific design, as you should place it as close to the grill as possible. You can wrap it up in a black plastic bag if you are worried about it being seen/getting rain damaged.
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