I have a full-length stainless flex liner for my Napoleon 1401 insert in a masonry fireplace. My question is how to fasten the rain cap to the top clay flue tile which protrudes out the top of the chimney. The rain cap has a flange bent over the edges of the top of the clay, but I don't know what type of bit I could use to drill into the clay and what type of screw to use to securely fasten it. I will be sealing the joint with high-temp caulk, but I need to find out how to fasten the cap on properly. Thanks!