Well I just made a box that vents outside for use inside the vehicle. It was a pain to construct though, and I don't really trust it to stay air tight, meaning I might have to redo it in the future. It would have been a lot easier to just stick a hose onto the vent like they do on cars that have the battery inside the passenger compartment.
In the future I was thinking about mounting batteries to the frame, but I was trying to come up with a way to heat them in the winter. The easiest way would be an air duct connnecting my heated interior to the battery box. However then I have to be able to vent the batteries via a hose or something.
Anyway it seems like one of the main reasons people go with agm batteries is the outgassing issue with lead acid, a simple vent cap barb would solve that, though I can't seem to find one sold. They have water maintenance caps with barbs but they do not vent through the barb as far as I can tell, though I have never seen them in real life.
They do have those caps with the catalyst in them, but are those effecient enough to allow indoor non vented use?