Updated 8-21-19: I tweaked this just a little so that all the text is on vertical surfaces so that you can print it laying flat without needing supports and still get good looking text.
I also came to realize that when I'm out on a shoot I don't often actually put my dead batteries back into the holder but just throw them into my bag. So I've also added a second design that is meant to only cover the terminals and is only marked full. It's not as solid of a fit as the full cover, there isn't really a good place to clip onto the battery on the right side. It's not going to come off on it's own but also doesn't take much force for it to be knocked off, so there is a chance it might come off in your bag if the batteries are bouncing around. But I figured why not, it's a nice compact design that protects the terminals and helps distinguish a full from a dead battery (dead having no cover on it).
Just a simple cover for Nikon EL15 batteries that has Full and Flat on opposite ends. I wanted a single cover that could be used to indicate whether my batteries were charged or dead while also covering the electrical contacts.
Insert the battery at an angle slightly back from the front of the holder as there is a small nib on the inside that the battery groove slides into, so it gives a little better hold than just a pressure fit.