I use either double backed tape if it's flat
mounted on the wall, or in one case, I took a
flat piece of sheet metal about the width of
the detector but twice as long, bent it about
140 degrees into a V shape length-wise, then
mounted the motion detetor on that (using
double backed tape). I then used some
caulking to mount the bracket in the corner.
Since the metal bracket's kind of thin, it's
easy to bend it to adjust the angle that the
detector's aiming down at.
Aaron