Sony used to sell a 4-pack of AA NiMHs that came in a collapsible "key-chain" holder. It can hold 2 or 4 AA cells. Four in a 2X2 vertical orientation, the top and bottom halves (two 2-cell sleeves) slide into each other so it can also hold just 2. I've since repurposed it for other brands of batteries. Still though, it was worth the purchase just for the handy key-chain container. I use it all the time.
Doing a google image search on: Sony AA NiMH 2500mah
I found some images of it there. Perhaps you can use that to find a supplier. Most report out of stock or unavailable. Maybe you can contact Sony directly and get just a few empty cases from them. Here's an example of what it looks like:
Note that when carrying 4-cells, there's not much friction to hold the two-sleeves together (not much top/bottom holder overlap), so I attached a strong piece of elastic band ("goody" brand hair-tie) to the top and shortened it to match the length of the case. It rests in the indent between the two battery-channels. Effectively locking it closed when 4 cells are inserted.