Kent the poncho I use is an integral dynamics silponcho - it has a shockcord on the bottom, so when I rig it to the hammock that is "stretchy". if your poncho doesn't have that it might need something stretchy to rig it up with, and gather the end up.

Four pictures are probably worth a thousand words - this is on my old speer, you can just see the wadded up space blankets in the second picture inside the poncho - I have a CCF pad inside the hammock here as well.