Simple single AA battery case with screw cap. I'm using this for carrying a spare battery when going for a run with my headlamp, which requires a single AA battery. This neatly avoids any danger of shorting the battery or getting it wet, without adding too much bulk.
Prints nicely in PLA and PETG. ABS works, too (the one printed in black), but is a bit tricky, since the threads tend to curl upwards. PLA looks the best, but might not survive a fall.
Threads are M20x2. Standard M20 is 2.5mm pitch, which would work, too, but for batteries that are a bit longer than normal, where the tip sticks out of the housing, a finer pitch helps increase friction, so the cap doesn't come off accidentally. However, most batteries I tried (all rechargeable) fit snugly.
See https://www.thingiverse.com/thing:2940648 for a newer design with trapezoidal threads that's easier to print in ABS and available in different sizes.
Fairly painless print. Some curling when printing in ABS, but as long as both parts are printed with the solid flat surface down, even warped threads work okay.
Fusion 360. While most of the design is parametric, the thread is chosen specifically for AA, so it would take a bit of work to adapt to other batteries.