A method for manufacturing a member for purifying exhaust gas for an automobile exhaust line, including an enclosure, an exhaust gas purification unit arranged in the enclosure, and at least one element for supporting the unit. The method includes the following steps: obtaining a first quantity representative of a mass (Mb) of the exhaust gas purification unit; using at least the first quantity obtained, determining an installation density (dmounted) of the or each support element; determining at least one diameter (Denclosure) of the enclosure as a function of the determined installation density (dmounted); assembling the enclosure, the support element(s) and the gas purification unit, so as to obtain the diameter that was determined for the enclosure and the installation density (dmounted) that was determined for the support element(s).