Here's how to build a Kestrel Falcon Nest:
The Kestrel Falcon normally nests in the holes (cliffs, buildings) or in the Old crow's nests.
It sometimes occupies a spacious semi-open nesting box, installed at the top of a tree or building (at least 5 meters) but the nest must be watertight (water must not enter the nest!).
This nesting box must have a ledge, so that the eggs, and the babies the first few times, are not likely to fall. It should also be equipped with an outside perch, a surveillance station as well as a hunting in sight of the parents.
Put a small layer of gravel (or sand/earth) on the floor.