If I have a poncho or space blanket, I use it inside the debris shelter rather than on top. This is in part because putting a small space blanket on top doesn't cover much of the "roof", in part because I think they are more effective directly on you or directly over the sleeping bag (in the cold, at least), and partly because I often need to take the poncho with me in the day time. If it's just wrapped around my sleeping bag or poncho liner (warmer weather), I can take it with me more easily than if its integrated into the debris shelter. I also find that putting a space blanket or thin tarp on a debris shelter results in holes pretty quickly.
Just my .02