Device for forming containers from blanks made of plastic material, having a mold provided with a wall defining a cavity intended to receive the blank; a boxing insert mounted movably with respect to the mold between a retracted position in which the insert is retracted into the wall, and an extended position in which the insert projects from the wall toward the interior of the cavity; a plate, on which the insert is rigidly fixed; a support on which the plate is removably mounted; and a system for the quick attachment of the plate to the support.