Two of the most useful pieces of gear you can carry in the woods are the USGI poncho and poncho liner. Together they represent a rain shelter, a ground cloth, an improvised sleeping bag.
FM 21-15 (1977) says that the combination is good for down to 50 degrees F. as an improvised sleeping bag.
The Poncho Liner
As issued, the poncho liner has ties that are fastened by the soldier to the grommets along the edge of the poncho. The liner consists of two layers of quilted nylon encasing a polyester loft filling. The issue poncho liners from Vietnam until a few years ago were variations of Olive Green on one side and camouflage on the other, either ERDL pattern in earlier examples or the later Woodland pattern. They now are made in ACU digital, a pattern that I despise. US Cavalry (The Cav store to residents of Fort Campbell) used to sell a liner made of thinsulate batting, but a call to them this morning informs me that they no longer offer that. Ranger Joe's offers a conversion that include all-round zipper and head hole.
Veterans generally remember the poncho liner with fondness. After the movie Mr. Mom came out (circa 1980), the nickname "woobie" was adopted for the poncho liner, although SERE instructors rejected that moniker, calling it instead a "Wilbe," as in "You WILL BE freezing your ass off if you do not learn how to use this properly."
Despite the fact that ready-made thinsulate liners seem to be unavailable commercially, there is no reason why you cannot craft your own.
The secret to warmth is layering. I have a buddy who has used a "double-wooby" for many years now, using two liners instead of one, and a thin OD tarp as a combination ground cloth/tent fly. He cut head holes in the liners, neatly sewing them up with OD ribbon edging, so he can wear them tied together or take it off and roll up in it. For really cold (here in Alabama) nights, he packs two space blankets for additional heat preservation. With the addition of native insulation like straw or pine needles, he claims to be toasty warm with this combination, even when the temperatures go down into the teens, all in package much lighter and less bulky than a sleeping bag.
Another friend took a cheap thinsulate woodland blanket and made a Ranger Joe's style liner. He swears by this combination. (He also uses a thick OD Bundeswehr poncho as a rain shelter/ground cloth.)
I invite any Threeper comments on their experiences with ponchos & liners.