I've made em out of the insides of ac/dc adapters.
View attachment 5182761 Once you pull the metal housing and the bobbins with copper windings it surrounds out of the plastic casing, all you need is some wire and magnets ( 4 little neodymium disc magnets work a treat)
Here's one in cigar box git I made. I used 2 rectangular ceramic magnets
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Cost me a few bucks all up to make one. Of course, I'm not suggesting such a thing is appropriate for a proper bass. But I guess it kind of qualifies as roll your own. Or at least using something rolled for another purpose in a new context. Can't say I was the best craftsman to ever walk the planet though he he