I've taken the door panel off my 1987 560SL and there's no wood there, its just the cardboard-like material that comprises the door panel and of course the metal door shell....
thanks for your answer though, now I'm leaning towards getting one of these cupholders myself! If only I was sure the screws would hold it nice and tight to the door panel.
Network Integration Engineer
MCSE, CLP, CCNA