MEDER - Magnetic / Reed Switches
pick one. any one which is tiny, low current.
even radio shack sells em.
you can take any home door alarm switch and open the plactic case, behold a reed switch.
key is low current and sensative.
can be tested , with speedo out on desk, with ohm meter , flip to continuity and spin the wheel. must open and close.
it can be overwelming the vast types.
Typical pull-in sensitivities for commercial devices are in the 10 to 60 AT range.
AT range , you want Low AT.
5 AT is very good. (my best guess) AT= ampere turns. 5 turns 1 amp = close.
Mouser Electronics - Electronic Component Distributor
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key points, small, short, low AT. Normally open single contact.
70 cents.
if i was you id buy a bunch 1 each, from 5 "AT" to 20 AT and try them all.