I use clear plastic fabric (Walmart sells it as tablecloths and tablecloth protectors), cut so it's a square (or rectangular) envelope with a flap and heat seal or glue the edges. It's strong enough to last (I have one almost 11 years old that's in and out of my pockets all the time), and soft enough to not break if I fold an empty one in half. Just put the battery in and tuck the flap in. (It takes about 2 minutes to make one.)