I'm considering the option of building my own battery
box vented outside. There are a couple versions of this available online, but once you add shipping to Canada you are looking at $100 each. $200 for two battery
boxes seems a little outrageous. Most basic boxes are designed to go in an outdoor location, but with my present configuration I don't have space or tongue weight
allowance to do that anymore.
My thinking is that I can buy a basic plastic bin of correct size. I can then put a vent out the top with basic plastic tubing. There is some concern with battery
acid eating the bottom of the box. I can solve that by buying two battery trays and placing those in the box.
Alternatively I could make the box with plywood and seal it up with a coating of fiberglass resin.
The other alternative of course is one of the sealed battery types (agm or similar). The box is probably most economical in the longer term.