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.