I use fiberglass wrapped in plasic as the core with a couple layers of batting, and burlap covering it. I have had a canary next to one the traps for about a year, and it hasn't coughed yet.
DIY and fiberglass and safety
I have a friend of mine that built some bass traps and acoustic panels out of fiberglass and considering what he paid for the raw materials, they do a good job; however, I wonder how safe it is to have all that fiberglass in a listening room. Does anyone know if it is safe?