For a project I needed a case for my Raspberry Pi 3B that had room for an 18650 battery and USB-C charge board. The original maker of this model provided the Fusion 360 file so I adapted it for my needs.
It now has room for this charge board (press fit), and if you wrap it in something isolating you can also add a step up to 5V which is not strictly needed. The one I used is a very large one and will be replaced with a smaller one once I receive it. If you use such a step up please don't forget to set the right voltage with your multimeter!
I didn't include the hole for the power switch since there are so many options. Just drill a hole yourself on the right spot that fits the dimensions of your particular switch.
The author remixed this model.
Added room for 18650 battery and charge board.