Disclosed is a retaining device for retaining a number of plugs and for arranging on a wall passage. In some embodiments, the retaining device comprises a number of substantially identically sized through-holes. In some embodiments, a modular element comprises an outer interface region for receiving the modular element in one of the through-holes. In some embodiments, the modular element comprises an inner interface region for receiving one of the plugs in the modular element.