Insulating the liner will keep it cleaner
This is not so, there are solutions. It's why oval liners are sold or fabricated on site if the installer has the proper equipment. There is also a prefabricated, insulated liner called DuraLiner that is made for this type of installation. Its OD is 4 5/8" x 7 5/8" IIRC.Unfortunately given the size of the flu (6”) anything more than just the stainless liner won’t fit unless we get an oval. Pricing that out today but I read it’s significantly more expensive.