When I joined an RC aeroplane club in Australia it was required that you put a key into a special frequency board to let everyone know that you are operating your radio gear on a specific frequency. This prevents other pilots switching on and shooing your model down and you doing it to them.
here is an example: http://www.hooked-on-rc-airplanes.com/rc-frequencies.html
I dont know if this is just for Australia or other places as well.
I could not find where to get the actual key so I copied my friend's.
I'm quite a n00b when it comes to the scad file but it works.
I printed in PLA with a Prusa I2 with no issues.