Capt I know this conversation has come up before and there are some great DIY ideas as well as plastic containers available. My idea is use a AA or AAA flashlight. It works the same way and has the option of being used. I'll carry 2 flashlights using the same battery so I can switch out battery if I need or just use the flashlight. My 2c worth.
Alright here ya go Capt. This Advil container will fit either AA or AAA easily. As you can see from the amazing pictures there will be some room to spare. Maybe you can fill that space with something to prevent the cell from bouncing around as well as protect the cell within. The top closes very secure and I dare say its water resistant/proof tho I haven't tested it.