Abstract: A port assembly for a non-metallic pressure vessel is joined to a boss which is molded into or otherwise joined to the vessel wall. A bayonet type accessory mount is insertable therein for sealing passage through the vessel wall. The mount includes a simple semicircular locking snap ring with easily manipulable handles. The locking snap ring is configured for retaining and locking an accessory such as a heating element in any desired rotational position within the port. An o-ring is sealingly compressed in a radial direction between the mount and the port assembly. A specific application includes the positioning of a non-linear electrical heating element within a non-metallic water heater.