Interesting mod, I like it. Is there a way for fresh air to come in near the bottom as hydrogen exits near the top. Although I'm picturing the tornado inducing bathroom vent fan in my trailer. If I run the fan with all the doors and windows closed it creates a good suction. This could would possibly suck air into the trailer through a fresh air vent in the battery box and bring any fumes in with it.
Also have you thought about possibly adding some studs and wing nuts to hold the side of the box on? I'm just thinking that the easier it is to access the batteries the more likely it will be that they get inspected.
LinkThey sell these at Home Depot in the drawers of specialty bolts in located in the nuts and bolts aisle.
I wonder if you can somehow make a sliding tray to make checking the batteries even easier. Maybe some drawer slides laying flat on the floor with a piece of 3/4 inch plywood mounted on top to act as a sliding floor.