Hi Richard,
From what I can find:
Nacelle frame - Ash strutted with spruce, except where the wings are attached to the nacelles, where there are steel tubes.
Nacelle bottom - Plywood, braced in the front with wood and cable in the rearmost bay.
Nacelle front and top - Thin aluminium.
Nacelle sides – Doped fabric, braced with steel wire. Heavy landings could damage the nacelle lower longeron immediately above the forward undercarriage leg, so a laced fabric 'inspection' access panel was made on each side of the nacelle.
Mike