I 3D printed a "holster" that can hold a single LIon cell or 2 that are soldered in parallel.  That seems to be a pretty common configuration in laptop batteries.

Then I purchased a set of battery contacts from Amazon and some common LIon charger boards. Each charger unit uses a holster, a charging board, and one set of battery contacts. The holsters have a "spine" sticking out on one side and a receptable for it on the other side so that you can print as many as you like and glue them together side by side. 

I paralled the inputs to the charger boards so you only have to connect to one of them to charge all the cells.

The lid and catch are attached to the holster by drilling small holes in the corner and pushing through a header pin.