I am trying to do the same thing and I saw these on Amazon and was going to give them a try with two 18650's to power my Arduino Nano 33 IOT. https://www.amazon.com/gp/product/B071RG4YWM/ref=ox_sc_act_title_5?smid=A1N6DLY3NQK2VM&psc=1
There is also an example sketch on how to monitor the battery with a "voltage divider".