A disk of joined segments, having a water resistant sheet over the seams of joining, is mounted on a ring around the nozzle, and water tightly sealed against the ring by expandable rings. A circular portion of the disk that is concentric with the central axis of the nozzle is thinner than the central portion of the disk at the central axis of the nozzle, measured in solid cross section. The thinner circular portion is contoured for resistance to stress from pressure of fluid against the disk. The contour includes a concave annular outer surface of the disk, rising toward the center of the disk. At least one segment includes a beam oriented longitudinally with the segment, traversing and clearing the concave annular surface, and attached at each end to the segment. A plurality of bolt fasteners hold the disk to the segment. The bolt is threaded at the front end for screwing into the ring, and threaded at the back end for receiving a hold-down finger knob. The shaft of the bolt includes a spacer to help align the disk within the ring, and a wrench flat for removal of a broken bolt.