As others have mentioned, poncho liners run about the same weight and price and don't require an additional shell for wind blocking. I found that the no-sew version was good for me down into the thirties (I'd rate it into the forties for the theoretical "average sleeper") and that the sewn up version--with one layer of InsulBright added to it--was good down to twenty-five (I'd rate it to freezing for the average sleeper).
Dejoha has a pair of fine tutorials located here
(for the no-sew version) and here
(for the sewn up version).
Hope it helps!