Installing a Hearthstone Morgan insert. New stainless 6" liner from stove connects to existing 6" by 8" oval stainless chimney liner which is mortared in above the smoke shelf. The new 6" round liner fits inside the oval (overlapping for a few inches). What do I use to seal the gap caused by the slightly larger oval liner? (There is good access from a door in the rear of the chimney, just above the smoke shelf, that was cut in when the oval liner was installed.)