Has anyone successfully added a magnet to their differential plug? Should I use some sort of adhesive?
You can probably buy a plug with a magnet already attached that fits. Just watch the clearance inside.
Similarly, my 350Z transmission plugs did not have magnets originally. The plugs have a convenient cup on them and I found a hemispherical magnet that fit perfectly. The plug is steel, so the magnet sticks quite well, but I put some JB Weld on it for insurance.
What size is the plug? You can also get round magnets with a hole in the middle. Drill and tap the plug for a machine screw to hold the magnet on.
I definitely suggest adhesive or screws to hold the magnet to the plug.