I had solved this mystery quite a while ago - I guess while visiting Helsinki for the Geopoly caches. It seems, that for some reason, we never got a good timing to get to ground zero. But today, while walking for some multi-cache, we decided to do some extra meters to also get this box.
At ground zero, one area looked very obvious... and I was the lucky one to get the box. But having seen such models definitely helps a lot, to spot the cache.