Disclosed is a molding unit for the forming of a container provided with a hollow recess, this molding unit including: a mold provided with a lateral wall defining a cavity and provided with a pocket that comes out into the cavity; and a boxing device. The boxing device includes: an insert mounted in translation relative to the lateral wall between a retracted position in which the insert is retracted into the pocket, and a deployed position in which the insert projects into the cavity outside of the pocket; an actuator integral with the insert and provided with a piston; and an added sleeve, mounted in a complementary recess made in the lateral wall and attached to it, and in which the piston is mounted in translation.