Big-ass piece of cast iron with a little hole in it.
If you can convince them you know the end product I’m sure the king can point you to his metal workers and in a lot of cases come up with a solution.
Also, it’s labor intensive but I think you can beat it into wire or use some other methods.
If the king says to the blacksmith “make that copper long ans skinny” he can probably make it happen.
Is it really a risk if it’s a guarantee?
If you smell a chemical smell while that’s in your pocket you better drop your fucking pants and run, if you even get that much warning.
I intentionally made a swollen battery go off and that was fireworks level violent and it had a jet of pink flame that went on for quite a while.