So I made my own battery tubes.
Taking a cue from people who make their own model rocket engines, a simple paper tube rolled up with glue is the perfect solution. You can make tubes of any size if you have paper big enough, and you can paint or decorate the tubes if they aren't hidden to make them part of a greater design. Make the tubes thick enough and they can double as structural components for your project.
"D" cells are kinda large so I started with two sheets of regular printer paper. I used wood glue but normal white craft glue is perfect for this. I added a few drops of water to the glue to make it more workable (letting the water drip off a freshly dipped finger is a good way to do this). A cheap brush to spread the glue will keep your hands clean.
Step 1: Get It Together
The tape serves two purposes: First, obviously, it holds the batteries together so you can roll the paper around them. Second, it adds just a little bit of thickness so un-taped batteries slide in and out freely but without too much extra room to wiggle.
Pull the tape as you go so the batteries are pulled together with the tension. This helps make a solid roll to work with. If you're feeling ghetto you can use the batteries taped together like this, but that makes them difficult to replace so that's only a quick temporary solution.
Since I'll be leaving the batteries in the rolls until the glue dries I made two sets. More on that later.
Step 2: Rolling Your Own
Apply the glue SPARINGLY. Remember that there is a lot of moisture in the watered-down glue, and water makes paper weak and wrinkly. The glue is mostly to keep the paper in shape and add just a little stiffness, so less is more here.
The only time I did add a bit more glue was when starting the second sheet. I overlapped the ends about half an inch tucking the second sheet under the first. The water in the glue will help keep it from drying so work quickly without actually rushing yourself.
Step 3: Finishing Up
An important note: LEAVE THE BATTERIES IN THE TUBE. The paper is weak and squishy from the moisture, so leave the batteries inside to help it keep shape until it dries. Unless you were really sloppy, the batteries will not be glued to the inside of the tube.
Once the glue has set, you can gently push the batteries out. If you made a thin tube for double or triple "A" size batteries try using a pencil or dowel to push them out. You're done!
Making the electrical connections at either end is something you'll have to figure out yourself.