We constructed a hyperbaric test chamber from a 6 foot length of 10" steel pipe with 1/2" thick wall. One end is closed off with a flat plate reinforced with frames, and the other is fitted with a reinforced 1/2" thick steel hatch designed to open inward so avoid the necessity for a locking ring or flange bolts. Once closed up and filled with water the chamber will be pressurized with a pressure washer pump. The chamber will be able to withstand pressures up to 2400 psi, or the equivalent of 5600 feet of see water.
We are using closed cell foam, anti-fatique floor mat for the hatch gasket. If that fails then Rod Kluwe suggested "Garlock Blue" gasket for high pressure pipe lines.