Hi Mako!
In fact, it's very easy to make a custom gunsight. I use my own, custom, sights for JABOs, with references for deflection shooting, bombing, etc.
I use Paint Shop Pro 9. So you'll have to find the equivalent commands for your Bitmap Editor.
Let's take the "defafult.bmp", default sight, found in your [ ..\Aces High II\sights\ ] folder.
This is a 128x128 pixel bitmap. 8 bit, 256 color, bitmap.
I tried with diffferent sized squares and they worked, 256x256, for example.
Open this "default" file file in your editor, PSP 9 works with layers, i think PhotoShop also, create another bitmap layer, so you work using deafult sight as a model.
Use 255, 0, 255 RGB color indexes for background.
Note: Although BMP files do not support transparency, HiTech, cleverly uses BMP files for sights and for squads's nose art making them have a transparent background assigned to some color index.
We have a value for background, 255, 0, 255 RGB.
For the sight lines or references's colors, just edit the file color palette and change some unused color index to one that suits your taste.
When finished, save this "work layer" as an 8 bit/256 color BMP file, to your [ ..\Aces High II\sights\ ] folder.
Hope this helps, and also hope you don't use those special shure-shot sights on me...