The Roach motel did a pretty cool inside battery box.
Here's a picture of the roach motels battery box from that page.
I also built my vented battery box inside my bus at the stairwell. Since I am not qualified to tell you if that was a good or bad idea (I don't even have my batteries in there yet), I'll leave that to the judgement of yourself and others. I included a picture below, at Jack's advisal I relocated my computer fan to the other vent so it would push the air through rather than pull (to avoid a spark combusting hydrogen).
. In an ideal world my out vent would be above the box, but it's not. There are 3 main reasons I built my battery box here.
1. I want my batteries easy to access, clean and maintain.
2. I want my batteries in a similar climate/temperature as myself.
3. I couldn't figure out where to mount a battery box beneath the bus which would be in accordance of the prior two ideals, nor could I find the skills, courage, or ingenuity to do so.