Amazon.com: BUYYAH Chimney Cap,17" x 41"Chimney Cover, Adjustable Bottom to Fit Outside Existing Clay Flue Tile Dimensions 16.7" x 40.7" to 19.3" x 43.3", All Stainless Steel Structure, with Meshes, Bolts, Screws :
HY-C SCADJ-L Shelter Adjustable Clamp On Single Chimney Cover, Fits Outside Various Sizes of Existing Clay Flue Tile, Large, Black Galvanized Steel - Walmart.com