I used 0.2mm Nickel plate, with settings of 1ms, 1ms, 20ms and an old truck battery. Everything worked well. the only "blow outs" were where i missed the button on the +ve of the cell, or if I wasn't holding the probes flat against the nickel with Sufficient pressure. Otherwise it went very well.
Slotting was more effort than it was worth. The sheet welded just fine without the slots.
I was hitting 3 welds per cell, with a 4th weld at 90degrees just for good measure.
Kapton tape across all edges before applying the balance wires, then balance wires soldered into the space on the nickel plate between the groups of cells, then Kapton tape everywhere after.
Andy