There are all kinds out there, but this is similar to what I had in mind: Stainless Steel Line Clamps - Speedway Motors, America's Oldest Speed Shop Not so much the specific part as the general shape/design. My thinking is these would be less likely to be disturbed by the heat than the direct welded nut, even though a clamp like this would have a secured fastener so that you could tighten from the outside of the header as needed if thermal cycling caused it to loosen. The "tongue" of the clamp would overlap the cup shaped flanges on the baffle inside the header. If, with or perhaps without the insulation material, the baffle were a snug fit you might be able to get away with a single clamp at the current hole you have.