I am happy to discuss problems with "under the van" battery boxes. Accessibility, lifting/lowering battery, difficulty in cleaning terminals and battery top, difficulty in replacing cable ends... But if you want one I would think a box that has an access panel on the long side combined with a side terminal battery might be ok. We use slide-out trays for many of our vehicles but they are not right for most vehicles.
This is an example of a Ford OEM frame-mount battery box. Very difficult to work with:
GM did a little bit better:
On most of our vehicles, accessible battery or not, we have switched to something like this:
