I suppose this is a very obvious piece of equipment, and, I suppose there are many other brands making these. Frankly, I’m not going to look around to find out. Because I already have what I know will be my fave piece of gear for the winter months going forward. The Ignite Backside Heated Pad is a little wonder. It’s a 20W heating pad powered by either one of the dozen battery packs you have lying around (requires USB-C) or a 12V car charger (again, needs USB-C), with an easy to use control for heating and an auto-shutoff so if you stash it in your sleeping bag and fall asleep, it won’t run all night. It’s a comfy, dense foam pad wrapped in a soft polyester you won’t mind getting dirty too.
I’ve been testing it out the past few days watching the stars at night in my backyard, and it will absolutely go in the car come winter for trips to the snow since we don’t have heated seats in our rides. Simple, awesome. Big fan. They make a full-size one too, but I prefer the butt-only pad, since it works on a truck tailgate, a log, a bench, whatever.