Mine are outside year round and I live in NY and it gets pretty cold here. On one side of the turtle garden I loosen up the soil and add some peat moss to keep it loose. Then I put about 2 feet of dried leaves and on top of that about a foot of pine hay. (I go in the woods and find a dead pine tree and take some branches.) I cover the top of that side (looks like a cave) with a piece of plywood covered with pond liner. Just don't use any green leaves or grass as that generates heat. I haven't seen mine for two days now as the weather has been down in the low 50's. They are dug under, but might still come up if we get a warm day. If you need pictures, I can post some.
Yeah thats the problem i dont think they can dig 2 feet. And yes i would love to see pictures. Until today they have been in a little hole right under the surface...they couldnt dig any deeper because there is concrete or something under the grass in my yard in almost every spot.
Brunation is a serious undertaking! When I first started with Box Turtles the following book help me bunch: "Box Turtles" by Tess Cook. She explains lots of non-hole-in-the-ground-options. May help.