How big is your space? As 2 4cell packs would drop into a 400x300mm Euro type crate for under a tenner?I have a similar fall back arrangement. The small space under my car is more square in plan than 4 in line. I have 8 cells ordered. They may fit in 2 batteries of 4 like Harry and Phils layout but If not I plan each battery of 4 to be 2 pairs of cells joined by a busbar, with a heater pad between, the cell of one battery made of 2 pairs in an L shape, hopefully the join being by a standard bus bar. The 2 L shapes then fit together into a square around a core of timber to take up the small empty space in the middle. The whole 8 cells then shrink wrapped and put in an enclosure with their Bms's. As long as the 2 L shaped batteries have equal length positive and negative tails to each other I would think that will work as there is symmetry between the 2 batteries.
I would not have thought that the positives need to be the same length as the complicated negatives through the BMS. As long as the 2 batteries are the same overall resistance, it should work.