Better bag, lose the air mattress.

I tend to sleep cold, so I often use both a thermarest and a closed cell foam pad under me. I made a couple of simple web straps to go around them to hold them together during the night.

I once made a mummy liner out of a GI poncho liner. It seemed to help with the el cheapo bag I was using a the time. You could probably make one out of a fleece blanket also. But a better bag is easier...