A method and apparatus for forming a number of filler members. The apparatus comprises a surface model generator and an analyzer. The surface model generator generates a first surface model of a surface and a second surface model of a flexible surface. The surface and the flexible surface are to be mated to form a mated surface and a mated flexible surface. The analyzer performs a structural analysis using the first surface model and the second surface model to identify a predicted final shape of the mated flexible surface. The predicted final shape of the mated flexible surface is used to form the number of filler members to fill a number of spaces between the mated surface and the mated flexible surface.