Is this acceptable or is it not commended. I've seen a video where it was done with 2x10 and lightly putting pressure on it to reach to desired oval. I want to insulate but am unable to us the foil faced blanket do to size (ID of flue tile is 7x11.5) I was told that even the mix insulation with 3/4 of inch and since it has an angle the line will probably rest on some of the tile,with zero insulation. I do what to insulate. Which way do I go ? P.S I don't want to break out the tile to get the round to fit