I bring this up over concerns over the soft body lipo packs and that 18650's are a bit unruly for space and 14500's tend not to have enough capacity.

Has anyone profiled the hard flat pack lithium batteries often used in eg. ps3/ps4/ps5 gamepads for use in place of the softbody unprotected lipos?
They are often rated from 1000mah to 3000mah with 0.4a constant current draw and some higher depending on which gaming peripheral it is from.
They also already have leads unlike some cellphone batteries and I haven't ever heard of anyone's hard pack cell in their gamepad flaming out.

The other idea I had was camera batteries and taking a cheap camera battery charger dock for the sliding body to work as retainer.