1. Use a CCF pad and keep it rolled up on top of your pack. It will also be your sit pad. Cheapest and lightest option.
2. Use a short inflateable, and use a GG nightlight for your lower legs. Keep it rolled up on top of your pack, It is your sitpad during the day
3. use a full length neoair xlite, and use a 1/8" thin foam pad under it to protect it. Keep the 1/8" pad rolled up on top of your pack during day to be your sit pad also.
4. Use a pack that incorporates a removeable pad on the outside (GG, Zpacks, etc) that serves as a sitpad.
5. Just take an extra small section of foam pad of some kind, and use it. But then its extra.