Good work, and fun huh?
I made my divider just like that and added insulation to it.
That works great for the package tray too.
For covering I glued 1/4 foam to the fit tested board and then covered it with black speaker face material.
The existing 5x9 cutouts are covered by the speaker material, with speakers on the underside of the tray metal.
No indication of speakers can be seen in the back.