The Thinny - 3mm (1/8 inch) Foam Sleeping Mat

You couldn't sleep on this thing but that's not what it's for. It provides a bit of extra insulation under your mat in winter, it can be a sit pad or (again in winter) you can roll it out on the floor of the tent so your not sat on the cold ground.
It's a cheap, light, versatile bit of kit. I'd reccomend getting the large and you can always cut it down if need be to make a little extra sit pad/cushion etc.

Ive bought several of these as they've become a must have on my ventures. I usually hike carrying two of these; one under each of our sleeping inflatable mattresses. These reduce the mattresses sliding around as well as boosting the insulation and providing a little mechanical protection against punctures. When I say 'we' I mean myself and my hound. I also use as day sit mat to provide a dry something to sit on as well being surprisingly warm to park my backside on. The beauty is that these wipe dry / clean so easily and don't absorbamy water.

This is perfect for adding some extra insulation to your sleeping pad. It’s also a good sit pad. Reasonable price and well shipped. Love what these guys are doing

The Thinny is simply a piece of cell foam which has proven to be a very helpful and versatile addition to my hiking/travelling gear. Used it for different purposes: as a (surprisingly comfy) yoga mat, as protection under my sleeping mat, as a seat cover, as a pillow between my knees sleeping on the side (rolled up a sweater in the mat), as protection inside the suitcase for fragile goods...
At the same time is super lightweight. Very happy with the mat!