To put batteries inside a laser cut enclosure, one has to think in 3D using 2D:

The pictures in this article will show how:

https://arduinoenigma.blogspot.com/2020/01/how-to-put-battery-compartment-inside.html

And now that there is a built in battery compartment, then the question of how long will it run on batteries becomes more important.

The following article shows a method of measuring run-time by writing timing checkpoints to EEPROM, no external components needed.

https://arduinoenigma.blogspot.com/2020/01/measuring-battery-run-time-by-writing.html