I second the Best Friends bareback pad! I just got one and I love it...I would stay away from anything with stirrups attached. It will cause you to slide around a lot and it will put pressure on the horses withers...it doesn't distribute weight like a saddle would.
I ride my horse mainly bareback...neither of my saddles fit him anymore so I don't really have a choice until I find one. But I'm tall and pretty thin and my horse is built fairly well (although he doesn't look it in the awkward angled photo below), minus his back being a tad long, and he doesn't have any back soreness issues. I think he actually prefers riding bareback. I am a "sticky" rider and don't move much at all when I ride...If I do, it is because he startles himself or bunny hops over jumps lol. I don't see any harm in riding bareback consistently if you are a quiet rider and aren't bouncing and sliding around all over your horses back. I always check his back after every ride to see if he has any discomfort, and so far so good!
This is the only picture where you can see my bareback pad...its cheetah! There is a newer version which I originally wanted but the online stores were completely sold out and the one I ordered took over a month to SHIP it so I cancelled the order and got this one instead