A hearing aid includes a shell (104) manufactured by a rapid prototyping process, so as to include a first part customized to adapt it to the shape of the ear canal of a user, and a second part that defines an aperture; an electronics module; and a plug (200) with means for retaining it in a fixed position in the shell. The shell has guideways (212) for guiding lateral portions of the plug to support the plug in the aperture. The shell is adapted to receive by the aperture the electronics module; and the electronics module is adapted to engage a lip (206) of the plug to secure it in the shell. The invention further provides a method of assembling a part of a hearing aid.