Case for SwitchBot Smart Lock Pro battery. Just use a small Lithium-ion battery with 1S BMS and you will have a working battery. I used salvaged vape battery + TP4056 for one and salvaged handy fan battery (already had BMS attached) for another. 

For the terminals: cut a short piece of Nickel strip, thread it through the slits and twist it a bit so it stay in place (use attached pictures as reference). You can also use a screw, but screw head protrude longer so I've prepared a shorter version for that use-case. 

This will function the same as the Pro battery: when battery A run out, it will alert you and use battery B so you won't be locked out (and vice-versa). It can also run on 1 battery while you charge the other.

Note: If you use TP4056 module like mine, change R3 to reduce charging current as these small capacity lithium-ion battery would not like that 1A default charging current. I set mine to 0.2c which is safe for all of them.