I have seen sheet metal heat deflectors inside behind wood stoves, with air space behind to protect wall. These would probably rust outside. You could take a piece of 1/2" cement board (like you use to mount tile on) and put that between your grille and house. It would be best if you could mount it with an air space between it and the siding. If you don't like how it looks you could apply some tiles or those thin fake bricks that you can buy at your masonry supply locally.