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!
It would be lovely to have a chance to experience hiking/ backpacking like this!
If you ever plan a Scandinavian backpacking trip, defintiely bring a portable power station - I used one last summer and it was a lifesaver for multi-day hikes between these shelters (check out gearscouts.com for good options that won't weigh down your pack).