My first and cheapest solution was to fit a second cheap hammock, like a Byer, loosely under the one I was sleeping in and fill it with a folded Poncho Liner:
http://i13.photobucket.com/albums/a2...k/99.07012.jpg
Alternatively, I did this with my modified Poncho:
http://i13.photobucket.com/albums/a2.../239.07008.jpg
http://i13.photobucket.com/albums/a2.../239.07007.jpg
http://i13.photobucket.com/albums/a2.../239.07006.jpg
http://i13.photobucket.com/albums/a2.../239.07009.jpg
http://i13.photobucket.com/albums/a2...3009.07007.jpg
http://i13.photobucket.com/albums/a2...3009.07018.jpg
Works surprisingly well, but is definitly not extremly lightweight.
If I wouldnt use and therefore carry along a Poncho Liner, I would have bought a cheap synthetic-filled rectangular sleeping bag and cut off the zips and superfluous material.