I want to share my design to carry extra battery and memory cards for M8 and M9.
It is a mere strip of leather 52 x 150 mm approximately, to which a foam with the hollow corresponding to the protrusion of the battery and a plastic box of an SD card are glued (the double-sided tape is OK).
The closure is mede with velcro.
I think the photos explain it better
Best regards and Happy 2018.
chema