Sorry if this has been covered before but whats the method in repacking rear silencer and what material is best?

My guess is it is very similar principle to a Rev3s?

You can buy fiberglass packing for motorcycles or find some house insulation and break it up into small pieces and stuff the canister around the perforated tube and stuff the plastic cap a little so you can get it fitted and before you fit it snug to the canister use a something like a pencil to stuff the end cap tight with packing. First time is a bit of a trick but once you figure it out the job is pretty easy.

Use the proper silencer packing stuff as it is nicer to work with. It's cheap as chips anyway and made for the job.