Here are the grille blocks I made
This is the area they'll cover
I used the lid to an old plastic storage container for them cause they were getting thrown out. It was pretty simple I just traced the outline with a marker cut them out to get a rough shape and kept test fitting until they had a good fit with no gaps. Then I used a utility knife to smooth off the edges and sanded them so they were ready for paint. One coat of black spray paint on each side and that was it. They aren't perfect but you don't notice them once they're on so it doesn't really matter.
They use the same mounting as the plastic grille itself does but that's only one side so I'll probably glue the other side on and then use caulk to seal around the edges for the best fit since there is some flex.
I guess we'll see how it works.