Not satisfied with aftermarket head unit install - fill putty & plastidip it?

I used the Metra head unit install kit to install my Kenwood Excelon KDC-X997 head unit. Everything works/sounds okay, but I hate the way it looks. I originally hated the piano black finish so I sanded it down to a matte finish. Now I'm really unhappy with how it looks. I can tell that it's sanded and can't get the sand streak marks out no matter how I try to finish it. I also hate the gap between the head unit install kit and the stock trim. I'd like to get some plastic fill putty to put in the gap so it looks continuous and then cover the whole thing in plastidip. Has anyone tried this yet, or know whether or not it will work?

Thanks!