Hi Jim,
If you are using flat packing, do not pack them too tight. There are many videos on you tube which will help you. I recommend watching a few before completing yours.
I recommend some high temp resistant silicone to be applied upon assembly.
Apply a light smear to the outer round section of the baffle and insert so that the rivet holes line up without trying to move the baffle around inside the canister. You can mark a line on the baffle and casing to help you line things up before final assembly. You can wipe the excess silicone off with a damp cloth once they are together.
Leave both canisters overnight to allow the silicone to cure. You can also apply some silicone to the flange where the canister connects to the end of the expansion pipe.
This will seal your canisters if you are running rich and prevent spooge coming out.