Like my SOTA activation mentor, Ignacio EA2BD, I also use a small rectangle cut out from a cheap camping sleeping mat, the one with an aluminum side. In fact, mine was a gift from Ignacio, so it's very likely coming from the same mat.
I also have a tripoded light aluminum foldable camping chair like the one shown by Ignacio in his post, but it's usually in the boot of my car and I have only used it very few times on some drive:on summits.
Most of the times I seat on rocks, some times, when available, I've sat on logs, sometimes on the trig point base and sometimes on the ground, grass, sleeves or whatever it's down on the floor.
My low back has very bad memories from an activation with snow on the ground without anything to properly seat on.