How wide is the oval? Why will it not work exiting on the 13" (larger) side?
If i remember correctly, the rigid round Duarliner is a 7" outside diameter?
so fill in current thimble hole,then cut new hole in wide side of chimney .This would cut into different walk way & take additional elbows & more stove pipe to get it back to where the stove must sit 3/90's
inside diameter of tile is 6-1/2 ''x11-1/4''