I made this set-up for my portable ice shelter a couple years ago, to power 12v LED lights, a small fan, GPS, charge a phone, etc. Inside is my lawn tractor battery along with a couple pieces of styrofoam for padding. It fits perfectly inside a Sterilite 7.2 Qt. locking lid container.
I wired an in-line fuse to the positive side of the battery, through a LED lighted rocker switch (mounted on the lid), to a 12v power receptacle and a 2-pin plug that mates to my LED lights.
A velcro strap around the whole works keeps the lid on, just in case.
This set-up could also be used as an emergency jump-start battery, or as a stand-alone battery to power boat electronics (sonar, GPS) to save on main starter battery. Small enough to stow in any storage compartment.
I did the same thing in a plastic ammo box. I used two sealed batteries from a Vexilars in parallel. Also added a 12 volt lighter plug to charge a phone and a few sets of plugs for led lights, vexilars and camera in my Clam. Works great.
I don't think it is very safe to have a battery in a non-well-ventilated area is it, especially when charging?
It's portable... so I take it out of my ice fishing sled, pop off the cover and hook it to a portable charger.
If similar set-up were used in a locker, same would apply. Or just open the locker lid while charging.