WilliamC
Thru Hiker
Something like this under your NeoAir can make a lot of difference to warmth.One surprise was that it felt like it was actually the Neoair Xlite that seemed to be the weakest link. I'm a side sleeper and when using the pad in warmer conditions have had it inflated quite soft for maximum comfort but it seemed like it was around the hip area where I was feeling some cold from the ground. I will be trying it inflated more firmly next time in cold temps.
Edit: It looks as if several of us have similar ideas. And +1 to @Robin's point about a balaclava. When it gets cold, you want about as much insulation on your head as you have in your quilt. There are some Apex balaclavas around, and Aegismax do a very warm down one for a cheap price, but you will probably need to modify is as it's huge.