Scandinavia often has these three-walled cabins available on a first-come, first-served basis. In Swedish, they're called vindskydd, or wind shelter. This particular one is northeast of Umeå, Sweden. No guarantees on what they're called elsewhere, but I have seen them in Finland as well. And I have heard of but not seen of them in Norway. In general, the freedom to roam is quite strong in these three countries as long as you are respectful and stay out of obviously private spaces like personal gardens or farm fields. Happy travels!
In Denmark they are called 'Shelters' but with a Scandinavian accent.
Although you just find them and use them, you can book them in advance. It would suck to hike for 15 hours just to find your plans ruined because someone else had the same plans as you.
They are very popular for new years eve too (if you have dogs)
https://book.naturstyrelsen.dk/