Have a sonicare toothbrush? I pop the neodymium magnets off before I throw out an old brush head because I save everything (drives my wife nuts). I would think you could super glue/epoxy one of these to your drain plug. They are sticky little suckers so the glue may be overkill on a steel plug. I haven't tried this and someone may give a good reason to never try it, just an idea.
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Bill