TL;DR: Use magnets to connect batteries
Most Batteries contain Iron or Nickel. Just use a magnet to connect the battery. On the other side you can use one of those cheap copper-clad cables from china or alligator clips
Bonus: The magnets automatically disconnect if they reach ~90°C in case of a short circuit.
I wonder if anything ever reaches 90°C with a AA NiMH battery shorted 🙂 But I assume this also works with 18650 LiPo(s) ?!
Yes, easily. I unfortunately tried to build a battery back out of eneloops this way. But my magnets are slightly too long and they connected 3 batteries with.