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. ( I’d eschew Mylar emergency blankets as they are thin and will not hold up to regular winter backpacking use.)
This article 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.