Well, let's see.

I did a google search. The first mount idea I came across is this one -
http://sheldonbrown.com/harris/lighting/mounts.html



Just rather than mounting it on the top of the...um, at the back of the stem (blanking on the word for the thing it's actually mounted to), mount it on that mount point on the rack.

2. But then I ran across a post from "tsl" - it appears he has your *exact* same rack, and just double-wraps the rubber band thingy -
http://www.bikeforums.net/showthread...=1#post7890077



(Follow the actual link to see 3 more pictures of how it's actually done).

3. And of course, you could just wrap some material (like rubber spacers) around that horizontal bar on the very back of the rack (right behind your trunk bag) and mount the light on there. Or probably could mount some cut pvc pipe on it in a similar fashion.