Short answer, a hammock. it's lighter than anything else that's anywhere
close to as comfortable. Other than an XL wide cot topped with two thick
foam pads, it's the only thing I can sleep on my side comfortably in the bush.

Having said that if you just can't stand the notion of hanging between trees.
you may wanna try something like the new exped downmat or Big Agnes
insulated mat. Some friends bought me a wide big Agnes for a present and
it's not as heavy as an old school air mattress, and relatively comforable. Dunno
if it would have the depth on it for you for side sleeping (doesn't for me, but
I'm kinda large.