Gelco Stainless 17"x17" base single flue chimney capI've got a very old, oddball-sized chimney (since I rebuilt it myself - I'm a mechanic, NOT a mason!) with no clay liner ... or any liner for that matter. Needed a cap to fit this thing. Hours of searching on the net brought me here. I had originally passed over this cap, since the specs said 17"x17" - my chimney is a hair over 18" in both directions - didn't think it would fit. After asking eFireplace tech support for an option, I was directed to this - I had no idea it would fit a wide range of sizes, due to the long set-screws used to fasten it to the exterior of the chimney. This is EXACTLY what I was looking for! Price was reasonable as well - a WIN-WIN!We burn anthracite coal to heat our home. Stainless caps are a MUST, as flyash from coal turns to sulfuric acid when combined with moisture. The cap has been up for about a month now. Does have some slight staining that I can see, but that's to be expected. It's very secure, so I won't have to worry about climbing up on the roof in the middle of a storm to place the cap back on after a gust of wind. This was a common routine with the old cap, which I had to cut and modify to fit INSIDE the chimney. VERY happy with this one. Those set screws have a point to them, which bites right into the brick real good. Also allows for easy removal for cleaning. It's well built, and should last indefinitely.