If you have the SL6, then you should have normal round bars, and can use just about any clip-on aero bar. I assumed since you mentioned the other Madone, you had the aero top version that is not compatible with regular clip-on bars.
So, you can look at any of the many options out there. The Redshift Sport ones are great, since they are quick release and can be removed and installed in seconds. I love mine.