A set of in-the-ear hearing instrument housing parts for assembly into a hearing aid housing, comprising a trunk part shaped for accommodation in the ear canal of a user and configured with a connector and configured for interconnection with a right tip part (12 R) or a left tip part (12 L), a plurality of differently sized first elongate members, each of which being configured for positioning outside the ear canal in the pinna of the user for retention of the trunk part in the ear canal and having a first end and an opposite second free end, wherein each first elongate member is adapted to abut part of the concha at the antitragus when the hearing instrument housing has been inserted in the ear canal of the user thereby applying a force to the hearing instrument housing towards the ear canal retaining the hearing instrument housing in a position in which the housing is pressed against an anatomical feature within the ear canal, and the first end has a left ear connector (60 L) configured to be attached to the connector of the trunk part in such a way that the first elongate member forms an angle with the longitudinal extension of the trunk part, when operatively interconnected with the trunk part, the angle being suitable for positioning the interconnected trunk part and first elongate member in the left ear of the user, and a plurality of differently sized second elongate members, each of which is configured for positioning outside the ear canal in the pinna of the user for retention of the trunk part in the ear canal and having a first end and a second free end, wherein each second elongate member is adapted to abut part of the concha at the antitragus when the hearing instrument housing has been inserted in the ear canal of the user thereby applying a force to the hearing instrument housing towards the ear canal retaining the hearing instrument housing in a position in which the housing is pressed against an anatomical feature within the ear canal, and the first end has a right ear connector (60 R) configured to be attached to the connector of the trunk part in such a way that the second elongate member forms an angle with the longitudinal extension of the trunk part, when operatively interconnected with the trunk part, the angle being suitable for positioning the interconnected trunk part and second elongate member in the right ear of the user, and a right tip part (12 R) that forms an angle with relation to the longitudinal extension of the trunk part, when operatively interconnected with the trunk part, facilitating accommodation in the right ear canal of the user, and a left tip part (12 L) that forms an angle with relation to the longitudinal extension of the trunk part, when operatively interconnected with the trunk part, facilitating accommodation in the left ear canal of the user, and a left ear battery door including the left ear connector (60 L) and a right ear battery door including the right ear connector (60 R). each first elongate member is adapted to abut part of the concha at the antitragus when the hearing instrument housing has been inserted in the ear canal of the user thereby applying a force to the hearing instrument housing towards the ear canal retaining the hearing instrument housing in a position in which the housing is pressed against an anatomical feature within the ear canal, and the first end has a left ear connector (60 L) configured to be attached to the connector of the trunk part in such a way that the first elongate member forms an angle with the longitudinal extension of the trunk part, when operatively interconnected with the trunk part, the angle being suitable for positioning the interconnected trunk part and first elongate member in the left ear of the user, and each second elongate member is adapted to abut part of the concha at the antitragus when the hearing instrument housing has been inserted in the ear canal of the user thereby applying a force to the hearing instrument housing towards the ear canal retaining the hearing instrument housing in a position in which the housing is pressed against an anatomical feature within the ear canal, and the first end has a right ear connector (60 R) configured to be attached to the connector of the trunk part in such a way that the second elongate member forms an angle with the longitudinal extension of the trunk part, when operatively interconnected with the trunk part, the angle being suitable for positioning the interconnected trunk part and second elongate member in the right ear of the user, and