Which handle do you have?
For the Tactical Carry Handle, you won't get perfect co-witness, but lower third if youuse an extra wide, low ring. Aimpoint used to include it with every optic, but has since opted not to. The Trijicon TA-14 (IIRC) will also work for that purpose.
For the A2 carry handle, the only way that I know of to acheive co-witness is to use one fthe adjustable goos neck mounts and whatever ring you want to use.
For the Dominator or Dominator 2, any ring that is .25 inches shorter than a standard co-witness ring will work...I like to use ARMS 22M68s on them, with the half-spacer generally.
I also like to use either a stand-alone rear sight or a flip-up rear sight with a Comp-M/ML-2 in the Aimpoint QRP ring mount with spacer, both mounted directly to the upper receiver.