My taillight finally arrived today and I installed it. I see what you mean about the ring, what a stupid way of fitting it. I just threw it aside and pulled out my trusty hot glue gun and glued it into the reflector, and covered the back of the circuit board with glue as well. Works great so far, really cleaned up the back end, especially since I got a smaller fender eliminator to put on in conjunction with it. Dark out now but I'll snap a pic tomorrow. I'll update if the light falls out without the ring, but I doubt it.
On a side note, it still bugs me how much movement the taillight assembly has on those rubber mounts. Seeing as it is all LED now which are a lot less susceptible to vibration damage, I'm thinking of just mounting it solidly. Anyone tried this?