I don’t think cardboard is a great option for a skeleton. It just breaks down mechanically too easily.
If you’re worried about the structural strength of the pack once it’s made, rest assured that once you finish the whole thing and put it in a big heat shrink tube and shrink it down, that makes everything a lot more sturdy.
Also the deck/enclosure helps support the pack as well.
As for actual electrical pack architecture, you should definitely be using something made of not-nickel (copper) for the series connections. Nickel is fine for making P-groups, but you really should have something like copper wire or braid or whatever for your high-current connections between P-groups.