foam mat roll, exactly like this, from harbor freight (or wherever).
cut it into squares or narrow strips, or micro(for easier packing).
why: save your butt. softer than sitting on rocks, gravel, or concrete.
benefit: cheap, light, effective.
cons: easy to forget (thus leave/lost risk) , often packs awkwardly.
*tip: large pieces double as shading tool eg when walking in the sun.
I've seen people use rolled foam like this rolled around the innerlr edge of their frame-less backpacks for a little support. They take up very little room this way, because it's just the thickness of the foam x however many wraps it makes around the inside of the pack. The "inside" space is used storing the rest of your gear, the added support is nice on your back, and it also protects the thinner fabric of the pack from anything sharp inside.