It would be hard to find many areas that don't contain trash. Try to find a can and punch some holes around the bottom (bottom of the vertical walls, not the bottom bottom) with your knife. Get a fire started with debris in the bottom, and gradually add twigs, fir cones, dried grass (twisted and knotted), etc. Set this little stove on a flat rock if possible (it will retain some heat), or on bare ground.

"You do not have much time each day to deploy your heating strategy."

If heat is important to your survival, you'd better MAKE the time. And don't wait until dark.

Sue