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.
Here's maybe a weird use for this material: If you get a completely smooth foam mat (instead of textured as seen in the photo) you can use spray glue and glue down a sheet of sandpaper. (Or multiple sheets of different grit levels)
Then, on the sandpaper, you can sharpen your knife by hand and give it a strong convex edge instead of a v-edge that you would get from a stone or plate. The "give" of the foam allows the convex shape to be formed in the steel.