A casing for an Elfbar 600 where you put a charging controller,that you solder to the battery.
Can be a very tight fit
I used these: https://de.aliexpress.com/item/32930640893.html?spm=a2g0o.order_list.order_list_main.10.483b5c5fD22p0x&gatewayAdapt=glo2deu
The author marked this model as their own original creation.