Sneaky trick for thems of youse what lets yer batteries die.
As long as its some kind of "button, or coin" type battery try this wizard wheeze.
Get a sheet of the old "slide file pages" or any similar multi-pocketed ringbinder page made of plastic. Cut out a few of the "pockets" individuslly leaving a sealed border of about 1/8".
Put a spare battery in the pocket.
Tape the opening shut & tape the sealed pocket to the outside of your stock with non-marring tape.
If you need a coin to open the battery compartment tape a second pocket with a washer or quater in it as well.
You'll never even know its there till you sudenly need a spare battery at midnight in the middle of a field. Then you'l be real glad you had it on there.