Try one that comes up to the middle of her back with a larger back pad it will give a more secure feeling in the twisties and more comfort on longer rides
also try preloading your rear shocks up a notch or 2 on your next ride with her
Congrats to you and the wife. I wish that I could get my wife on the bike, but she's firmly convinced that death awaits at every turn. She's right, of course, but I've managed to cheat him every time so far. That doesn't seem to be enough consolation for her.
My 883 will pull me and the wife at 70 plus all day long on the interstate. With that being said if I was going to make that (2up riding) a regular thing i'd buy a touring HD. I don't care what seat you have on a sportster, none will make her smile like a touring seat designed for touring. Thankfully mine doesn't like riding with me much lol ..