The different shapes is not the issue it is the fact that the stainless isnt tight in the clay so creosote falls down around the liner and needs cleaned out. Our fee for cleaning one like that is double a regular cleaning and may be going up again next year.It's hard to sweep when going from round pipe to square tile, I would think..maybe @bholler can shed some light. Heck, I think the guy that sweeps my neighbor's pulls his insert, and it's lined all the way to the top. That guy isn't all that sharp though..