Use this to make a custom bracket for holding the Parallax PING))) sensor. Print it at a 1:1 ratio and use it to mark the center of the holes for drilling. Currently untested! If you use it before I get the chance, let me know how it turns out!
PDF and Sketchup versions are attached to this page.
Note that the holes shown are based on Parallax's declared measurements for the actual features (0.32" radius for the "eyes" and 1/16" for the mounting holes). At the least you'll probably want to drill the eyes a little bigger -- an 11/16" bit should work and still clear the mounting holes.