I was struggling to find a 18650 battery cell spot welding fixture that was modular and flexible for the pack you need to build so I designed my own.

The cells are tightly fit in place to ensure minimal gap between the cells.

There is a gap on the bottom to make place for already welded batteries when you turn them around.

Each module slides into place, just make sure to thoroughly clean any support material that might be left, it's a tight fit by design.

In the third pic you can see that this modular design also allows for shifted battery pack designs, although there will be a bigger gap between the parallel cells.