Universal battery holder.

I use for 18650, but can be customized for any battery size:

• Battery diameter.
• Rows
• Columns.
• Space between cells.
• Holder height.

Note that the space you use between cells is also used for the external edge, so the holder is strong.
Other holders i tried are half thick in the external "edge" and can break easily.

Probably using cell_diameter = 18 for 18650 battery is too tight and the cells will not fit, depending on your printer settings you should add 0.1 to 0.5 mm.

There is an stl for 3S2P 18650 with 2 mm separation, 8 mm height.