This is the battery box for my boat. I added a voltameter and cigarette lighter to the top of the box. This is a group 24 sealed deep cycle battery in a group 24 sized box.
About the voltameter: It's waterproof with green LED. You can find details for it here.
The cigarette lighter was something I already had attached to the battery, just dangling outside the box. I use the cigarette lighter for low voltage applications (such as cell phone charging) so I'm not worried about heat. Just in case, I also installed a 5 amp, inline fuse. Come to think of it, I've never used it for anything but charging my cell phone.
Construction: I cut holes for everything using a combination of drill bit, hole cutter, jig saw, and file. I added duct tape to the holes and then poked the items in to seal the seems.