Head casts usually aren't flexible so you may have difficulties trying to attach them to a neck post. Drilling the hole wider than the ball is the only way to "pop" them on, but then obviously they fall off easily.

Hot glue is one way to attach them, but I've found that later on people complain about it because the hot glue tends to be super grippy. One hot day and is all it takes to slightly activate the hot glue and have it seize up the joint.

So, let's try Mod Podge instead (white glue).

Dab a little glue in the head's hole and smear it around.

Don't drench this in glue or it'll take forever to dry and harden.

Don't thin it with water, it just makes the MP runny and won't dry quick.

All you're doing at this stage is getting the head to rest at the height you want.

Tear off a little piece of paper and stuff it in there. Add a drop of glue and stuff some more paper in there until the desired height is achieved. Take the head off and let it all dry and harden.

Now the goal is to fill any empty space leftover, so use as much paper/glue as necessary. Just be sure that when you press the head on the neck post that the paper will squish out and under the ball, so it traps it in there once it hardens.

The picture here shows how much I needed for this head. Once I firmly pressed the head on, the paper squished out under the ball, trapping it.

And that's it, let it harden.

Turn the head like once an hour, just to make sure nothing is seized up. When the MP and tissue paper dries it becomes a tough semi-flexible material.

Since this was a Mattel WWE figure, I moved the head around to work the paper into a shape that allowed it to tilt and pivot as usual, then let it harden.