Abstract: An annular base having a central opening therein is provided for overlying and support from one side of a cover panel having a ventilation opening therein and with the base supported from the cover panel in at least reasonably good fluid tight sealed engagement with the portion of the cover panel extending about the opening. A cup-shaped cover is removably supported from the base and is somewhat resilient such that remote sidewalls of the cover may be inwardly deflected at the open end of the cover. Predetermined remote portions of the cover sidewall and corresponding remote portions of the base include coacting structure interlockingly engageable with each other upon return of the predetermined wall portions of the cover from inwardly deflected positions.