You looking for something like this;

B.M.A.S. Flat-Top Fixed Rear SightThis AR15 type fixed Iron Sight offers the option of mounting optics further forward on the flat-top rail. The sight’s base is a heavy, black anodized aluminum block - other parts are steel. Windage and elevation adjustments are 1/2 M.o.A.
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That's the one I got for My build, Thus far I have been real happy with it, and no problems.