They make that fiberglass/treated/metal infused wrap now that apparently works OK for this kind of thing. It will at least last till the products of combustion fill in the gap and seal it up. As Bones says it may smell for a bit. Cats get incredible hot, more heat than further down the system. I am pretty sure there is something out there you can use. I have also seen some flexible stainless with clamps on it that cinch it tight after wrapped. Might look into something like that too.