I'm no expert, but this method worked for me.
You want to get it as dense as possible, so try to pack it as tight as it will. Get some tape that will easily burn off when the exhaust gets hot. I used the kind painters use to mask stuff off. Wrap as much fiberglass as will fit in the pipe around the baffle. Then tightly wrap some tape around it to make it fit in your exhaust. After the tape I put some stainless-steel wire around the baffle. I then burned off the tape over the stove (be sure to ask the wife), and that left me with a good working baffle.
Don't bother leaving overhang in the barrel, this will probably blow out anyway.