Welcome to Wattflyer.
Your goal of a small, slow flying airplane that does well in the wind is a very difficult one. The smaller and slower the aircraft the more the wind will blow it around.
20 meters X 20 meters is a tiny flying space.
My best recommendations would be for one of the micro planes but they are not going to do well in more than 5 mph/about 8 kph of wind.
Examples would be:
The Ember RTF
Albatross RTF can handle more wind but may not be slow enough for that space
GWS Slow Stick - kit