If you attempt to make a plastic insert for your sight aperture, you might concider purchasing an oxy-acetlyene torch tip cleaner.... Get the one that looks like a metal pen and has bunches of little drill-bit looking files in it that will insert into a chollet in the end of the pen. Then essentially you have a hand drill that works well on small holes especially in plastic and softer metals. Works great for precise oversizing of holes for custom fit projects. (These things are invaluable when working with ATV's, dirt bikes, etc. that have carb. jets - jet not big enough - just slightly file it out a bit.) Get a small pilot hole in your new aperture, then file it out incrementally until you get the size you prefer.