An enclosure (20, 220) includes a housing (22, 222) and a sealing unit (48, 42a, 232) that mounts within a sealing unit opening (28, 230) of the housing. The sealing unit (48, 42a, 232) provides a peripheral seal between the housing (22, 222) and the sealing unit (48, 42a, 232) and provides seals around cable ports (50). The sealing unit (48, 42a, 232) can be mounted to and removed from the housing (222) through the sealing unit opening (28, 230). The base (26) lacks a permanent retention structure (55, 155a) at the outer end of the base (26) for retaining the sealing unit (48, 42a, 232) in the base (26). A cover (24) is removable from the base (26) without requiring the sealant arrangement (52, 236) to be de-pressurized. A fastening arrangement (55, 155a) releasably retains the sealing unit (48, 42a, 232) in the sealing unit opening (28, 230).