Spot Welder Choice
The spot welder used last year for MSXII was a SUNKKO 787, a very underpowered cheap Chinese unit that produces lots of sparks while spot welding. We also acquired a SUNKKO 709A spot welder, capable of 500A for the welds, which is good for up to 0.2mm pure nickel strips.
K-Weld ~$250
https://www.keenlab.de/index.php/product-category/kspot-welder-kit/
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A spot welder project for personal use that I found recently. Some good discussion on the design in the build log and the comments. Gerber files are available on the site, and parts are not crazy expensive. Per some quick math in the comments, this seems to be capable of around 2000A.
Edit 2:
There's a lot more welders that have popped up recently. One that I am more familiar with is Julien Lemay's spot welder pen. Still the same timed pulse method, but putting it here for completeness.
Its also not too hard to design your own, so that's an option in the future as well (at least for an experienced electrical engineer).
There was another one I saw that used 72 N-Channel TO-220 FETs connected to a 2S3P stack of LiPos that I believe came from a car pack.