Mounting bracket for the SainSmart HC-SR04 ultrasonic transducer PCB (one version of a "ping" distance sensor). Two models; one generic and one to fit a servo horn that comes with the Hitec HS-311 servo. The board just drops into a slot and is not fixed in place with screws, so it can be quickly removed. Fits equally well the other way (sensors facing away from servo arm). Please note there are at least two different "HC-SR04" boards out there, I know one type from ebay has different dimensions by a few mm and would need modifications to the OpenSCAD file to fit.
I used 3x M2 screws as self-tapping into the 1.5 mm holes in the servo arm, first using a M2 drill to ream out the matching 3 holes in the bracket. You may prefer to adjust the size of the holes in the SCAD file, depending on your hardware. If you want a more rigidly fixed PCB, you may prefer this one: http://www.thingiverse.com/thing:100486