https://www.thingiverse.com/thing:5810807

This is an extension for the Walmart brand 'Hyper Tough' battery housing. I've sized it for an offset stack of ten 18500 cells which can be salvaged from disposable devices. Please note the the battery is 5 cells in series, so your batteries should be 5 cells in series and the three packs in parallel.

For larger batteries, the bottom of the model can be pushed down. You may need to get creative for batteries larger than 18650s.