I would stick with Wool. Although you don't want to spend over $100, it's really worth saving up for a 5 star, tod slone, classic equine, impact gel or twenty x pad. (Wool pads!) They last a long time, and are great! I got my Tod Slone pretty used but in good condition for 70 dollars, and it is still working completely fine a few months later. My horse has really high withers, and it fits his back perfectly, and my friend's horse has like no withers, and she has a 5 star and it fits her horse perfectly too!
"If your horse says no, you either asked the wrong question, or asked the question wrong."