Reflex finders are easy. What they are is a lens that images a target to an infinite focus. In other words, it is the opposite of a telescope in that the illuminated target is the source of light and you look into it through a beamsplitter (just use plate glass or plastic uncoated for a beamsplitter) You will know when you are focused correctly when you move your eyeball around and the image of the target doesn't move around in the sky. Go look at a Telerad that somebody has for what the basic mechanicals is.