18650 Battery Holders

Low profile for easy removal. Designed around these inserts (not an affiliate link. Just for help finding the right parts)

Amazon.com: VNDEFUL 30 Pairs AA Battery Positive Negative Conversion Spring Plate : Health & Household

Wire routes in the bottom.

I print these withe the wire routes down and no supports and it works well on simple Ender 3 printers.

I printed a few and soldered female header connectors to each pin, so that I can easily use breadboarding jumper wires to create different configurations (image in gray). 

I've also added it to project boxes, like the image showing the wire routes in green.