Say I put an oval liner in my chimney, ( cause my chimney won't fit a 6 inch round insulated ) I have to convert to round at the top of the chimney for the cap , correct ? Also if I want to add some height to the chimney ( cause it's only 13' from fireplace floor to top of chimney and am looking at englander insert) using I think class a double wall or triple wall insulated ? I guess my question is how do I make all these transions, do I use rigid oval in the chimney up to the chimney top and a oval to round to hook to the stove, (flex) and how does it go to round at the top and what holds the weight of all this up ?