The thread on sourcing foam got me thinking and hunting around - having some time after just having finished this week a 3-4 moth period of 12+ hour days 7 days a week - and I started thinking about cork - expanded cork panels - as the core for stressed skin panels - using ply skins. (No idea if a "corky" would ever be a possibility but possibly interesting appearance with fiberglass and epoxy over.)
Yes - about 5 times the cost of foam - but how much is the foam a part of the cost of a typical tear? $1000 instead of $200 in a $3000-10000 project? I don't know.
Not that its a big concern of mine but it is a much "greener" materiel with fewer documented health and fire hazards, and may be a little better acoustically. Hard to quantify.
I did find one link that has some info, which I have yet to thoroughly study. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5455286/