I tried to experiment with AA battery instead of 18650 Li-Ion. The design is slightly smaller and lighter, but I think it will be worse on the long run. 18650 can provide more power and it's easier to have step up and charger on the same board.

I recycle my 18650 cells from old Laptop batteries. Usually those batteries go bad just because of 1 or 2 cells only, while other cells are still usable.

Or you can always buy 18650 cells online if you don't feel like cracking up laptop battery or don't have one to spare.

My rough estimates are about 150mA while light bottle is on, so 18650 should be plenty for several hours of your night talks.