Remix from the "space navigator for notebooks" 2,67 inch. This is for "normal" space navigator 2,99 inch.

http://www.youtube.com/watch?v=94k3xcvUurg

If you have the space navigator for notebooks 2,67 inch se:

http://www.thingiverse.com/tag:spacenavigator

If your space navigator is 2,99 inch in diameter, just print it and mount it over the Navigator.