Windsurfer / Range extender for DJI Phantom 3 Standard. A few of the other ones i viewed attached at the top and bottom of the antenna meaning a lot of support material was required. This attached at the bottom of the antenna. I had to sand the hole a little to get a good fit (my printer makes holes smaller than it should).
After printing you will need to attach copper tape to the concave surface. I found 100mm wide copper tape on ebay - it is used as a shield for electric guitars.
Similar designs, people have stated double the range, but i have also read that windsurfers are not good for close range.