If you’re a fan of thrifty DIY substitutes for camera accessories, here’s a quick and easy one for you: you can easily carry spare batteries (and other items and accessories) on your camera or bag strap using two strips of Velcro.
These step-by-step photos by Instructables member hagay1974 show how it’s done. The loop strip is used to secure the battery and hook strip to the strap, and the hook strip is used to keep the loop strip closed:
The main thing is cutting your strips down to an appropriate size. They shouldn’t overlap themselves when wrapped around the battery. Once everything is in place, you can simply open and close the top flap to access the new makeshift accessory compartment on your strap.
DSLR Strap Battery Case [Instructables]