That's why you use sheets of bark from a fallen tree instead of dry leaves or grass to cover the shelter. The bark is a lot less prone to catch fire. In the areas I camp, there are often fallen trees that you can peel the bark off in sheets with use of a hatchet or machete. Any loose dry stuff goes under me for insulation. I usually take a wool blanket which is adequate to arrest any sparks that come from the fire. I don't build a huge fire, and being conscious of the wind direction and where you place your shelter can largely mitigate the spark hazard.

When my friends and I go primitive camping, each of us has a small canvas tarp about 7' X 9'. When there's four or so of us, we'll throw up a small wigwam and put the fire inside. One of those tarps and two wool blankets is usually adequate in pretty cold weather. If there is less than four, we'll pitch a three sided lean-to with the fire in the middle.

Tom