I wanted cheap and cheerful way of being able to pull preset focus depths on my Sony CX900 and came up with this. There are three pieces - an 'anchor' piece which sits in front of the focus ring (where it wont move) and two 'puller' pieces which sit on the focus ring.

Line up your two focal depths, holding the focal ring and sliding one of the two pullers to the appropriate position. Once set up you can go from your initial focus, to your second and third positions (the two pullers) - and then you can still pull focus back and forward between them (for example if you have two characters at different depths of field who are engaged in conversation).

Watch a brief video of it in use here: http://youtu.be/OLc7UguKMm4

Note: When setting the position of the two pullers you should do so using the same pull speed as you will do when filming. If you set them by pulling very slowly between the two positions, then pull the focus quickly when filming - you may miss the desired focus. This is a limitation of this particular camera (as far as I can tell) in that if the speed of your pull is faster than the speed of the motors moving the lense it will not 'remember' the total distance your pull has travelled and so the actuall lense shift will lag behind the distance you have pulled. If anyone has some tips on how to avoid this please let me know!