Here’s a simple and cheap AA battery holder/case that you can pick up in the health and beauty section at Walmart. You can use it to store extra batteries for your portable flash unit safely and neatly in your camera bag.
The label says ‘iGo 6 compartment Travel Case’. You’ll find it with the travel supplies like toothbrush cases etc….
Inside you’ll find a side with 5 compartments and the other with one large compartment. The large side is just about the right size to hold 5-AA batteries.
‘Unhook’ the doors on the 5 compartment side and pull the insert off the case by pulling straight up.
I used a small diagonal wire cutter to ‘snip’ the end of the compartments where they contact the sides of the case.
Once those are cut, wobble the compartments back and forth to break them from the bottom. You can then use the wire cutters to smooth up the bottom.
Once the compartments are cut out another 5 AA size batteries will fit on that side.
It’s a bit snug (certainly not a perfect fit) but the latch mechanism seems pretty secure on these cases and seems like it will hold the case shut just fine.