I don't know I didn't think of this sooner, but anyway here it is now....
Bots tin can works good at adapting to the diameter mismatch between our header and a Harley muffler, but it's tricky....
At work we use shims to fill gaps in certain applications. More specific, laminated shims, where you can peel them off until you get the right demension you need, you can cut them readily to fit the exact area you need to fill.
I see you can buy them online, pretty cheap and in different sizes....
In our application I think we could build it up almost like a tape....
I will snag some left over from work and fool with it. We even have titanium which really withstands the heat, aluminum would not work too good, maybe it would tho, not sure how much heat it can take....

I covered the header/muffler joint with 500°F silicone and then wrapped it in several layers of aluminum foil. It's held up fine for many thousands of miles.

Maybe I should also add that I glued the aluminum foil to itself with high temp silicone and then clamped it.