A quick, simple, light and very compact alternative is the reflective sun shields that can be bought in any auto parts store or auto parts section of a larger store such as Target.
Simply place the sun shield under your sleeping bag and/or pad setup (some people like the visor on top of the pads others like the visor underneath the pads) and get some precious reflected heat back in your winter setup. I suggest using a truck/SUV one as that size is larger and is a better fit for full length sleeping pads.
This article on on the Hennesy Hammock page explains the concept in much more detail. (Scroll to the bottom)
So, if looking for a little more warmth for your winter camping, consider going to the auto parts store as well.
Not only for getting some essentials for winter safety while driving, but also for a quick camping trick, too. More durable than an emergency blanket if you winter backpack or camp on a regular basis, too.
Quick, simple, light, cheap: A sun shield. It works.