Ah well, shame. Perhaps Cydonia could try? Brass or copper, nylon mesh or thin open-weave tissue (single layer), aluminium and a multimeter. All clean, all dry.
It didn't have to be flat. I could get a copper tube and aluminium sheet to work. If I wrap more tightly the Al sheet around the piece of tissue I get a higher voltage than displayed here.
However, I initially briefly electrolyzed one end of the copper tube in very diluted KOH to clean it up (I forgot I didn't have to; the shiny surface is partially visible). In the beginning voltage was in the order -500 mV (under a tighter wrap), then slowly started decreasing and is currently -8.7 mV.
The outer surface of the tube is definitely dry. I cleaned it with alcohol too, although I cannot rule out that conductive residues still remain and have an effect through the thin porous piece of tissue.
EDIT: if I manually hold the piece of aluminium with great pressure onto the Cu tube, voltage gets to 250–300 mV or so.
EDIT: holding the Cu tube with the other hand (with a nitrile glove) causes voltage to get more negative, so this could be also a matter of grounding and so on.