This is a sliding mount for the auto bed leveling sensor type SN04-N used on some Prusa i3 3D printers. This sliding mount is much easier to adjust than fixed mounts and is easy to make from commonly available screws. You will need a 1.375" long 4-40 screw and a 4-40 nut to create the lead screw. The mount is screwed to the bearing block using 4mm cap screws. The sensor is screwed to the sliding portion using the original 3mm screws and nuts. This mount can be adjusted by very small amounts making it easier to get your Z height just right for the perfect first layer. The design can be adapted to other printers by doing a new base mount for the base and sliding parts which stay the same.
For convenience I added another members sliding portion design for 12 and 18mm round sensors. http://www.thingiverse.com/thing:1354218
Update: Some people are finding that their sensors are not reliable when powered with 5V . I've attached a circuit that I use that allows the sensor to be powered with a higher voltage such as 12 - 24 V and lowers the output signal to 0-5 V for the input to the controller. It uses a blocking diode. When the sensor output is 12-24 V, the diode blocks the voltage so that the controller input is 5 V. When the sensor output goes to 0 V, the diode conducts and the controller input is pulled to nearly 0 V (actually 0.7 V).