I completed the two halves of the muff. I riveted the wings of each half together at the ends, after putting high temp (Copper RTV, 700 deg F) sealant between them.
Then I drilled matching holes through the flanges of each half and test fit the 8-32 threaded screws and self locking nuts that will hold the halves together on the exhaust.
Completed
Note the rivets and high temp (700 F) sealant on each half.