Moulding unit for forming a container provided with a reservation formed as a recess towards the inside of the container, this moulding unit comprising: - a mould equipped with a lateral wall defining a cavity that is the impression of at least part of the container, this lateral wall being provided with a housing opening into the cavity; - a boxing device comprising: o an insert mounted with the ability to move translationally with respect to the lateral wall along a transverse axis between a retracted position in which the insert is at least partially retracted into the housing, and a deployed position in which the insert at least partially project into the cavity out of the housings, o an actuator secured to the insert and equipped with a piston; - the boxing device comprising an attached sleeve mounted in a complementary recess formed in the lateral wall and fixed to the latter, and in which the piston is mounted with the ability to perform a translational movement, the sleeve defining a chamber subdivided by the piston into a rear half-chamber and a front half-chamber; - the recess, the sleeve and the piston are asymmetric in terms of revolution about the transverse axis; - the boxing device comprising a fluid circuit controlling the movement of the piston, which circuit includes at least one primary supply duct supplying the rear half-chamber, which communicates therewith via several openings in the sleeve, which openings are distributed about the periphery of the sleeve. - a mould equipped with a lateral wall defining a cavity that is the impression of at least part of the container, this lateral wall being provided with a housing opening into the cavity; - a boxing device comprising: o an insert mounted with the ability to move translationally with respect to the lateral wall along a transverse axis between a retracted position in which the insert is at least partially retracted into the housing, and a deployed position in which the insert at least partially project into the cavity out of the housings, o an actuator secured to the insert and equipped with a piston; o an insert mounted with the ability to move translationally with respect to the lateral wall along a transverse axis between a retracted position in which the insert is at least partially retracted into the housing, and a deployed position in which the insert at least partially project into the cavity out of the housings, o an actuator secured to the insert and equipped with a piston; - the boxing device comprising an attached sleeve mounted in a complementary recess formed in the lateral wall and fixed to the latter, and in which the piston is mounted with the ability to perform a translational movement, the sleeve defining a chamber subdivided by the piston into a rear half-chamber and a front half-chamber; - the recess, the sleeve and the piston are asymmetric in terms of revolution about the transverse axis; - the boxing device comprising a fluid circuit controlling the movement of the piston, which circuit includes at least one primary supply duct supplying the rear half-chamber, which communicates therewith via several openings in the sleeve, which openings are distributed about the periphery of the sleeve.