I found that the single-beam laser was not entirely "stable" in terms
of where it hit the secondary mirror as I rotated the focuser (and I still
have not solved this mystery). So, instead of centering the secondary with
the laser, I used a cheshire collimation eyepiece (far less susceptible to
problems caused by slight tilts). Centering the secondary in this way was
quite simple,and resulted in this image when looking through the focuser.