Quite simply, get a large piece of art that you love and place it on the mantle, lean it against the wall, covering the hole. I do this and don’t even have a hole like yours! In the meanwhile…I would look for a place to match your brick so in the future you could fill the hole, or at least block it. But I would see this as a wonderful opportunity for art, and an interesting mantle display! (Pretty brick, by the way!)
Q
Love it all!
Q