The way SMB set it up in my van was two separate, sealed boxes. One was vented to the outside, the second was vented to the first via a hose and a hole cut in the side top of the first box. Both boxes had big holes (like 2"+) cut through the floor, presumably for drains? Both boxes also had a ground wire for each battery running through the drain holes. It was kind of a weird setup, very messy, and took up a LOT of room under the dinette.
I added a third battery and created my own battery enclosure, getting rid of the sealed battery boxes. You can read about it here: viewtopic.php?f=14&t=9376
My home-built box is vented and fairly well sealed, but I didn't worry about the drains or anything to protect the floor, as I'm using AGM batteries and so don't have to worry too much about overflow, spills or the like (fingers crossed).