The default nesting location of Long-eared Owl / Waldohreule (Asio otus) are abandoned nests of corvids like Carrion Crow, Hooded Crow or Eurasian Magpie, as well as other large nests (e.g. of Common Buzzard, Grey Heron etc.). Artificial nesting aids, like open platforms or baskets that imitate such larger nests are readily taken – see photos below of a natural nesting situation as well as a wooden platform that I installed at St. Martins Lodge/Frauenkirchen, Austria.
Much rarer is the use of half-open nest boxes for Common Kestrels. In the years 2019, 2021 and 2022 such a nest box at the main entrance of St. Martins Lodge, right above a busy roundabout, was occupied by Long-eared Owls and chicks were successfully reared. In each year, kestrels would use the same box to breed right after the owls, usually on day one after the last owl chick left in early May.
Also in the Seewinkel region, Long-eared Owls regularely breed in the half-open nest boxes made from plastic canisters, pictured below, occupied by kestrels.