You can buy these connectors. But printing them is often faster. 

There is a two cell and a three cells version. Connect as many of them as you like, creating any battery size that can be larger than any print bed. 

The female side of the connectors has a slight chamfer, making it easier to connect its counterpart. Connect the male part from the side that has the slightly wider opening. It is still a fairly tight fit. So you will need to press with your thumb or some tool and with your body weight on each dovetail connector to make it fit flush with the surface. 

UPDATE 2024-11-29: I updated the design to make it easier for the parts to slide together. 

I printed it with two perimeters and 10% infill with ABS. PLA or PETG should work just fine. 

I used it for building a 6p6s LFP/LiFePO4 battery with 500 Wh.