[Cobranut]

Welcome to CC.

If the inserts are perforated tubes, you'll definately want to wrap them. Otherwise, they won't do much muffling.
As far as the material, I'm not sure what to use but I'm sure someone here will be able to help.
I've heard of some muffler makers using a stainless steel wool or mesh. I'd think it would resist breaking up and blowing out better than fiberglass.
I've had glasspacked silencers on a couple of bikes, and they eventually got very noisy as the packing disintegrated.

Later,