Vessels, or housings, and covers therefor are detailed herein. Loads and load-bearing surfaces exist principally at the periphery of the covers, by contrast with centralized loading utilized in other designs. In some versions of the vessels, a cover is sandwiched, at its periphery, between a cap assembly formed of both a jack ring and an upper component or cap and thus carried on and off the associated vessel when the cap is placed onto or removed from the vessel, respectively.
