My apologies, it seems I have left questions unanswered here...
For working area I intend a limitation of the local airspace where pilots can either do sightseeing or practice manoeuvres/procedures/aerobatics without the risk to interfere with other aircraft.
At the location where I am currently flying there are 5 areas that can be assigned for practising, since the corners are not perfectly identified by natural points or characteristic structures, it becomes of course important to have a precise visual clue on a moving map.
I tried the Skydemon tool for PC, but it is not really easy nor intuitive to use. Moreover Skydemon team do not provide instructions to define polygons as the feature is not supported by their development team.
I am still open to smart suggestions...