Kewl....very neat idea....but again...gel is way to go now IMHO
Gels are good no doubt, especially in such a location as being discussed, just mo-munnie! That little device, although added expense, is less expensive than the extra cost of gels.
Their use is especially good for multi-battery systems, such as golf carts, and/or hard to reach service locations.
The kewl thing is, they're self leveling and won't over-fill.
Here's an idea for the rail mounted battery boxes. Why not install a step down hinged battery tray, that swings out and down? You know, kind of like those hide-away door steps. Of course battery cables would have to be a few inches longer to accommodate movement away and down from the rail, but would make R/R, terminal service, and if wet cell- service a load easier. Since it's so close to the rail, a small 12V service light could be mounted for those dark times too. Direct wire to battery with wire pigtailed for stretch, fused and either a manual switch or pin switch- weather proof of course. Maybe a rock guard in front... I know, getting wacky again.