A structure for an instrument or device, such as a hearing aid instrument, having a shell for enclosing the instrument components and an opening through a wall or face of the shell for access to the components wherein the shell has relatively large dimensional tolerances while the components are to be mounted within relatively close dimensional tolerances. A unitary or multi-part interface frame for mounting the components into the shell is mounted to the shell, typically in the opening therein, and has an outer contour adapted to the dimensional and shape tolerances of the shell and an inner contour adapted to the dimensional tolerances of the components.