We had a similar project to accomplish for Wilderness Survival merit badge back in my Scouting days. We had to build a shelter in a pine forest and so we propped large branches against a tree, ran smaller branches between the propped ones, and then used handfulls of pine needles to fill in the gaps for waterproofing. It worked pretty well, since we got some steady rain later that evening. Incidentally, we used all deadfall because that was all we could reach. These pines had no live branches lower than 50' off the ground.

The only nature that was disturbed were the bees in the nest that we uncovered in a pile of needles. So we also got to demonstrate our First Aid skills.