Don't think attic insulation, fiberglass comes in many forms. They use a very fluffy version of fiberglass that looks for all the world like raw cotton. A little hard to get ahold of, though. Like I said, any high-heat fabric will suffice (kevlar, fiberglass, pre-ox, etc). And if you're changing the size, their exact configuration may not work for you. If you produce a larger fireball, You also produce more heat, maybe longer burn times, etc. In which case, the dynamics of the tool change.

I think your simplest way to learn about them is to get some 1" kevlar rope, cut half diameter pieces (with a little extra on the ends), unstuff the ends about an inch or two in, then stitch together. They'll burn up in a couple of burns, but you only need 4' of rope to figure out where your weak points are (and if you can handle the heat).