It 3mm Okoume marine plywood. Not bending ply, just very thin. It's hard to do anything with a full sheet it's so floppy. Thought about tying it in a knot and taking a photo. But this worked just as good.
Before you ask Okoume ( o-KUE-me), also known as Gaboon, is an African hardwood widely used in Europe over the past century.