Try a perforated plate both prior to the muffler and at the end of the muffler. Cut out four circles - one front back each side.
They will reduce the noise considerably. Expect a medium reduction in HP but they can be easily removed.
If you make the plates in a cone shape with the base of the cone the size of the inside diamater of the pipe and the cone approximately 4'' high it will reduce noise.
But they will not restrict the flow as the cone shape increases the opening for gases to move through. Again use the two cones each side.
I can assure you that the steel wool will burn up very quickly as I have been down that path finding rusty balls and fibres when I pulled my side pipes apart.
Bernie