I took this photo when I first got the set, to record how things were arranged on the neck:
The fiber gasket with big notches is used to jam the focus coil in place. (It's between 5 and 6 on the ruler.) That's in the neck zone where you'd often see an ion trap magnet.
The focus coil is between 6 and 7 on the ruler. It is a strong permanent magnet, so I don' t know what would happen if an ion trap magnet needs to slide close to it. The trap magnets are much weaker in my experience.
In this photo, the focus coil is too far forward (CRT-face-ward). In practice, I have moved it back toward the tube base -- maybe another 1/2 inch -- to get normal focus.
Between 7 and 8 on the ruler are the centering rings, also permanent magnets. The phenolic sleeve that lies face-ward from the rings is attached to the yoke.
I can probably come up with a 10BP4 and a magnet to try. I guess I could leave off the fiber gasket temporarily; otherwise the magnet might not be loose enough to slide over it.
Directly under the CRT neck is an adjuster in an aluminum can with a hole on the top. There's only about 1/4 inch of clearance between that can and the CRT neck, so the magnet hardware might collide with the can if it nears the tube base.
Phil Nelson