Summary

Commercially available battery boxes do not fit protected 18650 batteries, which are a couple of millimetres longer because of the protection circuit, so you have to make your own. This box can be used with springs from
https://www.amazon.co.uk/dp/B09XHGDZYZ?ref=ppx_yo2ov_dt_b_fed_asin_title
which just slot in, and can then be soldered into your circuit. The batteries are held securely in place, but can be removed for recharging off the device, which is recommended strongly!
The box was printed on a BambuLab P1S in PETG.