U552 wrote:It looks like if the O rings are installed right just on cylinder edge and the covers presses them against to make it watertight?
Yes that's correct. The module diameter is larger than the vast majority of cylinders used in the U.S. The module is constructed from acrylic tubing, which is usually extruded. Although dimensionally very accurate on the outside, extruded acrylic tubing tends to be a bit wavy on the inside- you can usually confirm this by rubbing your finger around the inner circumference.
As a result getting a reliable seal with a small cross section seal like a 3-4mm thick o-ring can be a bit of a gamble. However it's easy to seal against the edge, and as the design of the ballast system in the OTW module pressurizes the forward and aft cylinders, andrequires tie rods to hold the endcaps together, you might as well make additional use of them by axial sealing.
The OTW module is likely to have it's edges trued on a lathe, but you can also cut the tubing square and smooth the edge with wet and dry paper used wet with a little soap on a flat sheet, and work the tube in a figure-of-eight pattern.