Hi Jurek,
Contact Eagle Protection Commitee in Poland. Their site is in Polish but they speak English.
http://www.koo.org.pl/kontakt-top
They put artificial nests and improved existing ones for WTE for two decades or longer. The current population in Poland ca. 1000 bp.
Interesting site, thanks a lot for the link! I found the page on artificial nests, sounds (from the automatic translation) like their experience closely matches what I've read about the German eagles.
Apparently, the lack of suitable old trees protuding above the average level of the forest canopy is a problem for White-tailed Eagles, and in Germany, they are more likely to pick trees at the edge of a forest, or even isolated trees, and often they choose trees that are not strong enough.
I've found two or three mentions in the German press of platforms being built off-season or in nearby (more solid) trees, without an artificial nest. In one article, it was mentioned that "starting" a nest was often successful in enticing the eagles to "take over" the beginnings and make the nest their own.
In other cases, it seems that the eagles didn't like the man-made improvements and instead built their own nest in a less stable position - in one case, right next to the artificial platform.
It seems that in one case, eagles newly settled on an island in the Elbe river, and only found a willow to build their nest in. The nest apparently came down at least three times, though in one case the young survived and was raised by the parents on the ground. (The island is off-limits for humans.) The provision of a an artificial nest is under consideration now.
Regards,
Henning