1) as others have said above, let some air out of the mattress to make it more comfortable.

2) Have you thought about getting the best of both worlds? Get a lightweight air mattress and put a thin foam mattress on top. You could do that for under 2 lbs. The foam mattress on top, could give you some rigidity that is comfortable for you and the air mattress would give you the thickness. If you have a problem of them slipping against each other, you can paint on thinned out silicone rubber. and they should be pretty stable.