Stoney Point used to make turret's like that. Had a set on a rifle years ago, and they worked great. I just checked my suppliers, and don't see them listed anymore. I'll dig some more and see if I can find you a set.
Active LE Sniper, AR15/M16 Armorer, Firearms Instructor, NRA Certified Instructor, Taser Instructor, Sniper Team Leader, and owner of www.bigrivertactical.com