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home depot has sheets of aluminum for a/c ducting.. rivet to plastic undercover to cover that gap. then just attach the aluminum to the bumper via clips or bolts. you can also get nutcerts or clipbuts to install onto the underside of the bumper for a more secure install.
Mine blew out so many times after i installed c-west n1 front bumper, I made so many different adapter pieces to fit it correctly. In the end I just removed it for good.