An insert (2) for a hearing device is disclosed. The insert member (2) is a flexible sealing part configured to be inserted into the ear canal (12) of a hearing device user. The insert member comprises at least one through-going hole (46) permeable for sound generated by the hearing device, the sound being transmitted through the insert member to an eardrum (16) of the hearing device user or the at least one through-going hole being configured to allow an acoustically active part of the hearing device to be releasably fastened thereto. The insert member comprises a bag structure (6) containing a non-hardened shapeable material and wherein an opening (22) for receiving a hardening member (24, 26) is provided in the bag structure (6). The non-hardened shapeable material is configured to be hardened by means of a temperature increment and/or electromagnetic radiation, such as UV radiation or a chemical reaction.