So I made this on the fly after finding an old project from 15 years ago. This video is a little longer than usual as I wanted to show all the steps of making a double biquad.
A double biquad has a gain around 13 to 14dbi if constructed properly. Now if you replace the flat reflector with a parabolic curve you can increase this to 15 to 16dbi. Template: https://drive.google.com/file/d/0BzmJxxD-Z4B6dHdqUVhnUWpqYmM/view?usp=sharing