Putting some putty to fill the gap can definitely work. If you have a dremmel than you could just carve out the right sized hole you need bit by bit.
If not than you can fill it in with the putty, put some plastic wrap (or a scrap of a plastic bag) over the ball joint, then put the head on where you want it. That way the head should still be able to turn and move when the putty dries.
Mal's suggestion is great, too, but I'd be a little worried that, given the place where Sledge's head joint is, that it would be too easy for the peg to break off.
I'm looking forward to seeing him when he's all said and done.
