a method and appertaining system load 3d shape information defining a hearing aid shell into a processor and present a representation of the shell on a display . a fill boundary is entered by a user operating a user input device and a fill region for the shell is thus defined . the fill boundary is then displayed , but can be modified by the user . various checks may be performed to ensure that the fill region is a proper one , and if not , a status can be provided to the user indicating why the fill region is unacceptable . the displayed shell can be rotated and moved to assist the user . once an acceptable fill region has been defined , an indication is provided by the user that the displayed fill region is to be used as the actual fill region .