Inspired with the ongoing flurry of magnetic ground probes, here’s my take.
I took a male->male 3.5mm headphone cable and cut off the plug at the -end- of the housing. This didn’t work well using a female end and just attaching the magnet to that. I used a cheapo I found on amazon. I chose it because it was cheap and had a braided nylon jacket that I thought would hold up well.
Filed the end flat. Now the end is solid metal, and flat so there is something to attach things too.
Glue a single layer of aluminum foil to the end, and glue a magnet to that. The foil does a good enough job of bridging from the conductors that are left to the magnet (it’s a little wrinkly and pokes through the glue). Superglue is too brittle, the magnet snaps off after a couple uses. Conductive epoxy would be best (but I don’t have any). Regular epoxy is a little fussy, but seems to work well enough. I’ll see how it holds up.