I dont know if its the right way to do it but i used cotton insulation i got from my local motorbike shop but class works as good, i cut the full length of the centre section and rolled it severaltimes so its really thick i also wraped some wire around it to hold it so when you stuff it back in it doesnt move.
My muffler is different from the muffler pictured, mine i lenthened the centre section about 3 inchs and made a new perforated tube for the cotton to wrap around and its super quiet.
the origional can and an off cut of the tube i made

After, since this shot i welded the front cap on.