DrDiabolical wrote:...some sort of thick white material...
Ethafoam. ("Effin foam."
) Probably polyethylene foam, but I'm no chemist.
I'm told it is used for packaging. The above mentioned guys in Arcata get it from Wing Inflatables, which is a manufacturer of inflatable boats and pontoons, located right there in Arcata and themselves active in KSR. But I don't know if the foam the guys use is left-over packing material or actual product material.
Ethafoam deteriorates from outdoor storage, but is otherwise close to indestructible. Easy to carve with a knife and does not crumble.
If you have difficulty identifying the stuff locally, I can mail you a sample.