GoPro battery case. Holds 2 GoPro Hero 7 style batteries. Simple and lightweight (7g). Positive retention of both batteries, with even just 1 battery it doesn't slide around in the case. Also includes small windows if you mark your batteries.
I suggest printing 2 in different colors: one for hot batteries, one for dead batteries. As you replace them, put them in the dead case to keep them separated and identified in the field.
Inspired by Gopro battery case For HERO5,6,7 by @Sheepsfeather
1mm walls, so an extrusion width of 0.5 works great. If you leave the defaults it should work out ok as well.
Not sure why Thingiverse points to a bogus link on Amazon for this filament. Here's the correct link: Atomic Filament Perfect Orange Opaque PETG. Great filament!!