I filled a smaller (than the Bulls Bag) shooting bag with similar lightweight material. If I remember, it was those Styrofoam beads used in beanbag chairs. Maybe it'll work for you, but I found the material to be too light to give me good, firm support.I should be getting my rear bag soon in the mail and have been thinking of using perlite, put in a plastic bag first of course then into rear bag. Light and no yuck factor..