Do you have a pic?I vent by convection so there is an open space at the bottom of my battery box and a vent leading from the sealed top. Consolidating the hydrogen is the concern so I would not trust bottom venting. The box will be full of hydrogen before the venting works. Diffusion will help but why take a chance. Just leave the batteries open if you can't top vent- the explosion will be smaller then!
The way you describe makes me picture it having to go to the bottom before venting back up again.
Which would be exactly the same as a hole in the floor.