Walker Acousti/Seal applied to the edges of the patch.
(Keep away from actual hole, within reason. It expands like Great Stuff (maybe not as much as Great Stuff does) when heated by exhaust and fills gaps, etc.)
It will mushroom out a little, and fill the gap between the patch and the pipe inward from the edges.
I had used it between head and cat on a late 80's VW. Heat is no problem for it.
RTV will rely on cleanliness and thoroughness to seal well. Acousti/Seal is way more forgiving.