Now, as the winter is over (the second one, since I asked my question), I found some kind of solution:
I built a "tipi" from conifer twigs (For all foreign people - like me: conifer are the group of trees with needles instead of leaves, not like in for example German , where "Konifere" is one particular species)
You should choose a native European species, to ensure your rabbits can try a bit and stay safe. But if they are well fed with various kind of foods, you do not need to worry about them eating the conifer twigs. They would only try, if they are starving.
Sadly, I missed taking a picture of the building itself, but I will include a sketch and an additional picture of the "porch" I built onto their regular hutch.
