I have a very light weight pencil barrelled upper with a fixed front sight on my flat top AR. I have a Trijicon ACOG TA 45R-4, which is one of the "Mini ACOGS", (actually originally designed to mount on a fixed carrry handle). It is a simple crosshair with no BDC, and a LaRue Mini DOS QD LT 105 mount. Problem is that with only 1.5 power magnification, the front sight is much too visible through the ACOG. I want to keep the fixed front sight along with my Troy BUIS. There is only a crosshair, i.e. no bullet drop compensation on this model of ACOG, so raising it up enough to see over the front sight doesn't concern me. What kind of riser block can I use under the LaRue? Or, more importantly, is there a better suggestion? Thanks in advance, Jerry.