A coin cell battery holder parameterized for the number and size of cells and wall thicknesses. Print to hold the cells at the bottom of their slots by friction, and push them up via the bottom slot and then grab the sides from the top.

This design is chunkier than some but it is going to live and work in a toolbox so the robustness is a feature.

If you want to build a multi-cell battery look here http://www.thingiverse.com/thing:267438 . 30 volts in the size of a D-Cell!

One end is marked with a hole to keep track of relative state of charge or whatever.