I found a seller in the US who has lithium cells with built in protection circuits. I think 18650 and 4/3A are the same - I remember I found him while looking for cells from my own Lombard. They're only 2200mAh though, and no solder tabs. Cheap though.

Conductive glue with strips should work, or if the image above is correct, maybe they can just drop in. Maybe. I am not a battery expert.