I carry a closed cell foam sitting pad (approximately) 30cm x 40cm (12in x 15in) in the back(inside) of my two day packs. I use this every time I stop just to make breaks more comfortable and my posterior dryer.

In a Survival Situation I would supplement it with the day pack itself. These are all backed with closed cell foam. Between the sitting pad and my bigger day pack I have a fairly good mattress.

I would place these above my Four-Finger Bowbed and/or any other biomass the forest might have if their is no evergreen bows.

Improvisation keeps us comfortable and alive in the wilderness and a mattress can make the difference between descent sleep and no sleep.