Remixed 18650 battery cell case as shared by @betterForm.  Holds 7 cells.  Changes include softened corners, chamfered top + bottom, 18650 marking in the lid.  Two sizes of base - one at min possible size (as shared by betterForm) and one at a dimension that matches the lid.  Purely aesthetic difference.  At my print settings, the smaller bottom section saves about 6g of filament.  

Pictured example printed at 0.2mm layer height, 0.4mm nozzle, 116g of Hatchbox PLA for lid + larger base.  Printed nicely, with some light corner lift on the lid.  some artifacts as internal parts of the design get VERY close to the external perimeter.  Didn't bother me enough to add additional size/mass.