An apparatus comprises a distribution conduit, a diffuser body, and an insert. The distribution conduit defines an aperture. The diffuser body defines a mating tube, at least a portion of the mating tube passing through the aperture, and the mating tube defining a mating tube passage therethrough with an inside sidewall. The insert is at least partially disposed in the mating tube passage and presses against the inside sidewall so as to cause the mating tube to expand. Removal of the mating tube from the distribution conduit is thereby inhibited.