A cover assembly for covering a vessel comprising a main cover comprising at least a first outlet opening adapted for connection to a pipe, and a second opening surrounded by a flat region adapted for placement of a gasket; an access cover comprising a flat region adapted for mating with the flat region of the main cover; and provisions for the sealing of at least one cord passing between the main cover and the access cover. In other embodiments, the cover assembly is provided with an engagement feature adapted for engagement with a hook-shaped hanger rod. Sewage basin assemblies are also provided which incorporate these cover assemblies.