Don’t put the rings in your pocket, either. Your pants are not a good magnetic shield.
You could substitute any 8051-style microcontroller without much difficulty at all. With a bit more difficulty, you could even use an Arduino Pro Mini.
None of this triggering circuitry is quite what you want, but it’ll get you started in the right direction…
Given that the transformer is free, getting one that can supply a kilowatt for half an hour without breaking a sweat seemed like a Good Thing.
The general idea is that you can get nearly all the spendy bits by gutting a kilowatt-class microwave oven.
Much as I expected, there’s just not enough energy in a 5 V / 200 A resistance soldering unit to weld 8-mil nickel strips to AA cells.
Prudence dictates checking for end-to-end continuity after you finish soldering and before you do the Kapton + epoxy thing