BTW, When I said use JB Weld, what I meant was - Glue the original piece back in with JB Weld. Don't try to just fill the hole with it.

Clean it all good with alcohol first.

Once the piece is in place, add a blob of JB Weld on the underside of it for extra strength. Just keep the muffler side clean for a good seal with the muffler.

If you get some JB Weld on the seal, wait till it's cured and then chip or sand it off rather than try to wipe it off which could make the piece move out of place - unless you can easily clean it without the piece moving.