I got a cooler-type bag from Walmart for $11 and mounted it with j hooks to the rear carrier. I drilled holes through the bottom of the bag and used the hooks to bolt it on. I used some aluminum angle strips I had laying around on the inside of the bag, then a washer, lock washer, and nut on each hook.
As you can see, it's a very crude setup, but hey, it only cost around $15 for the whole works. Now we'll see if I ever use it.
More pics in my gallery, page 16.