Hey that's a great Idea, I never thought about bottle caps.
I'm not sure if I posted it or not, told Bruce and Ghurd about my PVC battery sticks for the Ni-cad cells, may have posted it?
I take a PVC pipe that the batteries slide into nicely (Stuff I had laying around) and put a cap on both ends. I round a screw or bolt end so that it's smooth and won't cut a hole in a battery.
Drill hole in center of PVC caps to screw in the screws or bolts, fill stick with cells, press on the caps. Tighten the screws just enough to hold all the batteries tight together and to make good contact.
A battery stick!
Adjustable voltage, just cut the PVC pipe sections to different lengths to fit more or less batteries. Use the section length you need for the amount of cells you need for the volts you want.
If you have a good spring like used in the base of flashlights you can use that for tension to hold the batteries togther.
Normally I do not fasten the cap onto the pipe, press them on and pull them off, they seem to fit great normally and hold tight. When I do have a loose one, I drill a small hole for a little set screw in the cap, press cap on pipe and tighten the screw a little.
I found this to be a great fast way to make DC power packs, it should work the same with any round batteries using the correct ID pipe, AA, C, D, but I use it for Ni-Cad Cells.
Keep a few caps made up and around handy ready to snap on pipe, then just cut the pipe to fit the batteries needed. Need 3V or 6V for a radio or CD player or 12V for a fan, just snip off some pipe and load cells then cap.