Use this Flex Liner Deluxe Terra-Cotta Top Plate in order to mount the Top Plate to the existing Terra-Cotta flue. This Top Plate includes a hinged single flue cap. The liner fits in the hole and is secured to the top plate by tightening the hose clamp band in the inside of the top plate collar.
If you have the old clay Terra-Cotta liner sticking out above the chimney crown you have two options. You can break the old clay tile and make it flush with the top of the chimney crown and mount a flat top plate to the crown, or you can mount a Terra-Cotta Top Plate directly to the old clay liner.