I have seen TacomHQ's TARAC used for this, but it is sorta expensive. It works well though.
I have not used Warne's rings, but I have used Burris XTR Signature rings to get some extra slope in and it worked exceedingly well. It is not something you can adjust on the fly. Same for Aadland's Select-A-Slope mount.
With Warne, I am a little unclear on how the rings accomodate for different inclination with different ring distances, but I will not be able to see it until SHOT (it is on my list though). That would be my concern with any dual ring solution along with tube binding when you change slope. I am sure they accomodate for that, but I would like to understand how.
I have seen Matt Ivey's adjustable mount used for this with good results, but my personal experience is limited. Same for the ERA-TAC mount.
ILya