Use this tool to create an 18650 Cell holder.  Modify right here on Makerworld, or download and open the file in Autodesk Fusion. All measurements are in metric (millimeters). 

1. Change the number of rows and columns (smallest is 1x2)
2. Change the 18650 Cell spacing
3. Change the 18650 Cell opening (incase your cells vary in size slightly)

I also have a print profile of pre-made cell holders from 1x2 through 5x5 (two on each plate).