Also built LiIon packs as the Lipos started to loose capacity. As it is hard to get cells at the moment, I chose Molicel P42A 21700, they were available. The battery is split into two 6S packs to fit the hatch. Max I could fit in was 6S8P.
0.2mm nickel strips
It was a bit tricky to solder the 8AWG wires to the strips (between the cells) but I had to minimise the size to fit them through the hatch.
I tried to route the balancing cables in a way they do not cross each other or other P-rows as the ones they are attached to. Where I had to cross, I added extra insulation (0.3mm plastic sheet). Used holes in the end caps to route the balancing cables to the bottom to keep them away from cells.
Transparent shrinking for easy inspection.
They just fit in.
Thanks @kotnascher for inspiration and advice