I'm reminded that most hens of most species (if they aren't unhealthy) will lay and set if they have a dark, sheltered, unoffending place to lay. Personally, I have doubts the free-range turkey hens will lay in winter, it's seems their reproductive systems shut down when they 'know' that it's futile to try to reproduce. I would fashion a box; snug, but not cramped; with a with one end open, (4 walls, and a bottom, tipped on it's side) with a very low, small doorway; basically, you want the hen to think she is the only think that can get in there. Of course, I also suggest placing decoy or dummy eggs, to fool the hen into thinking: 'this is a safe place, otherwise, the hen that laid these wouldn't have laid here'. (chicken, duck, goose, etc. pretty well any large eggs will probably work) Hope this helps.
Banny