In case you're still looking, I found a plastic container with a screw-on lid (something the tennis ball container was lacking) at home depot that was full of assorted bungee cables. Since I needed some bungees anyway, I gave this a try for a battery housing. It's a little bigger in diameter than the tennis ball container, so I had to slightly modify my bottle cage but it worked great for the week I used it.

I've since placed the batteries in my rear panniers for now as I decided that I get too thirsty to give up a cage to batteries.