I am not sure about all the packing people put in, some use aluminium, others use epoxy fillers. The computers at ICP design the structure of the assemblies and the tensions on the sheets are part of the design, including the distortion of the sheets caused by the tension. All the fill and packing adds weight and changes the design. The ribs under the wing nose skins for instance are not designed to be smooth, nor the sheets on the ribs of the flaps and ailerons.