“Do you use a sit pad or something as frame ?”

If using an air core pad, I think you hit upon the solution.
For the Mariposa, I use the Sitlight http://www.gossamergear.com/cgi-bin/gossamergear/sitlight.html at 1.4oz. I am surprised how comfortable the pack is using this at 20lbs. A big comfort help was when I took the support poles out of the pack.

For this to work, it helps to have a pad pocket to keep the ‘frame’ in place.

-Barry