I've been working on / building a slightly different battery compartment. Primarily because of the annoying difficulty of finding similar parts and materials here in the UK, but also because I wanted them to be able to take a wider range of batteries / fit in a different ROV body.
Below are some photographs of what I've come up with so far - the bodies of the compartments are 50mm polycarbonate tubes (3mm walls). Obviously the 44mm inside diameter is somewhat wider than the OpenROV batteries, so I've made a shelf that slides into the tube, which has the advantages of having braces to support the tube against pressure when at depth, as well as allowing different sized battery shelves to be constructed to hold wider / slimmer batteries.
The ends are 5mm acrylic, and they take 3mm o-rings on both the front and back. I'm using steel rods to hold the end caps together (the design has holes for 3 rods) I suppose a longer o-ring could have been used to keep them tight, but I'm also using the rods as part of the ROV body, so they made sense.
These should / may be able to be retrofitted to the standard OpenROV body, attaching the bars on the battery compartment to the cross bars on the ROV with ties / other links.
I'd be interested in any feedback. Also, if anyone is interested, I'll be putting the laser cutter files for the end caps and shelf up on my github shortly (as soon as I've tidied up a few things :) ).
Quick note: In all of the above images the shelf is in the battery compartment upside down - the red wire runs along the top of the shelf in the cut outs you can see in the version at the front of the enclosed tube :)
My ROV design places the single leg at the bottom, so they would be correct for me when fitted. The internal shelf can be rotated so that the single or double legs can be at the bottom - in case anyone was wondering.