Just wrap the magnet in sandpaper:
Lay some protective tape on the outside of the guitar to protect the finish, then all you have to do is scrub back and forth with the magnet on the outside to do a quick and perfect job of sanding the most inaccessible interior surface.
That was a hot tip, Dan.
Here's another.
I keep some spare loose magnets around just to locate steel inside instruments. Here I'm able to sense the steel reinforcement inside a mandolin neck:
Regular magnets aren't always strong enough to do that.
And here, I can find the exact location of a hidden neck attachment bolt so I don't get a big fat surprise when I drill for a strap button:
