Why not fill up with loose cells? They are small, they roll. Couldn't they roll into a ribbed tray and then a rail on both sides clamps them in place and connects them? Or pulled in on a chain like a machine gun loading mechanism?
This is an original idea that I haven't seen elsewhere.
The big issue is that assembling loose cells into a battery pack is a hard enough operation. It must achieve good mechanical properties (the pack must be rigid enough), good electrical contact with the battery terminals (bad contact would ruin a pack, could start a fire as a bad contact could overheat) and good cooling (battery packs heat, so afaik all designs use liquid cooling, which needs very good thermal contact between the cells and the cooling assembly). Loose cells would essentially mean you're feeding the loose cells inside the pack and the pack has some robotic assembly arms that it uses to put the cells in the right places while it achieves the three objectives.
With some different, taylor made cell design you could make this self-assembly easier, maybe even make it mechanically passive - have the cells orient vertically and slide into their places, and then they're locked. But the current cell design is definitely not chosen for this.
