I got my ideal bareback pad at an expo several years ago. I had ridden in several and didn't like any of them, they all were either too thin, or unstable. I ended up getting this one,
El Companero, because it was on sale super affordably. I adore it. You can get it to work with a western or english girth. I got western because I felt like I could get it a little more snug. It has a mounting strap, but I don't trust it enough to have ever tried it out, I just climb up from something else. Hubby got one to, so we have a teal and a purple, and we've ridden them in all weather, rain, gone swimming with them, they have held up to everything, with no color bleeding or fading, and are super comfy. But the best part, because of the texture of the suede and the cushion of the seat, your rear end is as good as velcroed to the horse - it doesn't move AT ALL!
But the only down side is the cost. We got our two for less than you could buy one now. And I did have the back strings tickle one very sensitive pony.