I prefer diamond shaped points of aim. As well as some commercial targets I often use square blue post-it note taped to white paper. Rotate post-it note 90 degrees so it forms a diamond. Select post-it note and paper size based upon range and magnification.
As I work in an office all the paper I could want comes out of the recycling system.
Blue gives ample contrast against white and all shots are easy to spot, this is a nice frugal solution.
