Someone posted a pic of his bike with those mirror extensions installed, looked really cheesy. Those extensions for the stock mirrors actually screw into the little port on the levers that's covered with the black plug.
Here's the thread with pics of a bunch of alternatives:
Mirror alternatives... what mirrors do you have
I was considering those extensions as a cheaper alternative until I saw the pic, now I'm going with the DP relocated mirror kit with the signals(which does come with the aluminum clamp), about $200 more than the extensions.