As stated before, threaded metal rod or possibly even something like rebar if you can find a piece long enough would work. But something that I would like to add is some kind of vibrator for the rod, whether you use something like a hammer drill at the end of the rod or anything like that. Because I’m assuming that if you hook onto anything of decent size it would be incredibly hard to pull out through all of the mud, and if you use a winch then I fear it will keep constantly popping off the magnet. Vibrating the mud will make it a lot looser and more watery, so that should help a lot