I used .090" acrylic form a local Home Depot window department and laser cut the holes to be a light press fit for my batteries. There's no magic to having six holes, it just seemed like a good number. The outline dimensions of the acrylic are the same as a credit card.
I engraved the battery type on the cache, just for good measure.
Now, as long as I keep the "Wallet Cache" replenished, I have fresh batteries at hand wherever I go.