If what you are looking for is a voltage applied across the scrotum, then a battery seems like a reasonable solution. Skin isn’t the worse conductor but if there is no complete loop I don’t see why it would be problematic and you could certainly boost the applied voltage.
If magnetism has anything to do with it then maybe a coil would be better, you could rip one out of an old motor or tv set and remove the core or rewind it into a suitably sized cylinder.
You’d probably need a serial resistor, with consequent voltage drop, prior to the coil.
As the reactance of an inductor (like a coil) goes up with frequency of supply maybe an AC supply source could be used - not mains - and the serial resistor avoided.