I redesigned this Battery box starting from the excellent job published by Peterpur.
I did my new release of the battery box to improve two aspects:

1. applied round upper and lower corners as they resulted a bit too sharp;
2. applied a guide for rubber band to keep box closed even if carried in a mountain bike backpack.
   I also modified external shape to follow batteries body, but it is just an aesthetic improvement with no other purposes.

Update of Sept. the 26th
I modified the box a little:

• modified side holes to elliptical; it will help on avoid overhanging when printing upper side;
• enlarged battery slots to have more internal clearance.

Cover and external dimension of the box remain unchanged.

I suggest to use the box puttin batteries with "+" sign towards the cover whether they are full; with "-" towards bottom when they need recharging.
This will help managing batteries.

I printed it in black PLA at 0,2 mm layer thickness, 100% infill.
100% infill makes very small difference with lower infills, but avoids printer shaking when extruding very thin infills.

You may want to try thinner layer extrusion, but I think that, because of the destination use, 0,2 mm is more than enough ;-)

Cover may result a little hard at the first two or three open/close operations; then it acquires the correct clearance to work smoothly.