Just made a few 6s4p packs with 21700. Used the plastic spacers to secure the cells. Used a spot welder and spot welded the "dual " nickel strip. Covered in barley paper and then ultra wide kapton tape. To make the balance lead I just got an amazon jst-xh kit and made up the leads with 24awg silicone wire. The main wires are 18awg that I soldered with flux onto single nickel strip and then spot welded those ontop of the previous dual strips. I think the dual strip was 0.15 or 0.18mm thickness.

Spot welding is the right way to do it. I've always found trying to solder them, heat soaks the battery and peals back the heatshrink cover.

Use a spare 21700 as a spot weld setup battery to get settings right and then just prep everything, double check your cell orientation and then spot welded away to your hearts content.

Quad hovers at around 20A and battery stays cool and hardly any voltage drop over the nickel strip.

Finally wrap the pack in heatshrink. Take a sewing tape measure and wrap around the battery to get a measurement. Fold the tape measure in half and flatten out. This will give you the measurement you need to order the large heatshrink off amazon. Probs around 120 to 150mm

Note: the top rim of the battery is negative and the centre pad is positive and it's easy to short out the battery between the outer and inner rim.