This flange with an O-ring seal fits the tube for the non-locking version of the 3 Watertight Enclosure. Comes with O-rings.
O-Ring Flange (3″ Series) – Ocean Robotix
BlueROV2 Underwater Vehicle Kit w/150m Tether & 4 Lights - RobotShop
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A circumferential clamp and centering ring form the vacuum seal between the connecting flanges. The centering ring contains a rubber elastomer o-ring.
Centering Ring KF-16 Vacuum Fittings, ISO-KF Flange Size NW-16, Aluminum & Buna O-Ring (lot of 1pcs)
OSK™ Max Pool O-Ring Kit EPDM 70 O-Ring Kit 1415pc [MAX1381KITE70] : The O- Ring Store LLC, We make getting O-Rings easy!
O-Ring Flange (3 Series)
BlueROV2 Underwater Vehicle Kit w/150m Tether & 4 Lights - RobotShop
O-Ring Flange (4 Series)
Spare O-Ring Set (3″ Series) – Ocean Robotix
Distinguishing the Different Types of International Flanges