I also 'sleeved' mine in plastic, not only to look better than metal flex duct but also to help insulate and seal it. But I used 1" thick pipe wrap insulation under it first, and I did not come any closer than minimum distance to combustible allowed to the stove with the plastic wrap either. Still doesn't look as nice as yours though, but with the bare metal exhaust pipe showing I'm not too worried about looks for mine.
