I missed the beginning of this thread, so this might be unhelpful, but what about using a guitar pickup? (single coil obv) there are loads of cheap pickups out there that may be so-so for guitar use put provide a coil (or two) with 10k winds or more for not much money.
They are infamous for picking up EMI
remove the magnets, use a high impedance input and either pick one that already has iron slugs as polepieces or install your own ferrite core as the shape of the coil and bobbin permit?
they are quite sensitive to capacitance, so you'd want the electronics close to the coil and probably several Mohm input impedance.
