Since you already have a piccatinney rail, I would suggest this:
Burris AR-332 Prism Sight
They also have a 3x version, but similar $$$.
Link to Midwayusa:
Burris AR-536 5x 36mm Prism Sight Ballistic CQ Reticle Matte
And the lesser one::
Burris AR-332 3x 32mm Prism Sight Ballistic CQ Reticle Matte
Read the reviews....