Debbie
You mentioned that the saw dust is intended for your cob bench, actually the straw in the cob is not for insulation but for structure. Like fiber glass cloth in resin the straw works to hold the cob together. Maybe a better example would be steel mesh or rebar in concrete. Another thing is that you would not want to insulate the bench you want it to radiate heat out to your room, and cob with the straw as reinforcement works great for that.
I can't say for sure but about any kind of mesh material like an old lace curtain or fish net like material would work but I'm no expert so others may have better suggestions.
Hope this helps
Vern