Have you tried a pad with flow foam in it like an equipedic
or skito? Their foams will adjust to pressure and conform somewhat to the
situation created by the saddle and the horse’s back. I have not
had problems with either of them slipping. My training partners all use
them too. We all have different shaped horses and we all ride Advantage saddles
made for each horse. Right now we all have equipedic pads and use Dixie
midnights underneath them.
BTW, I know of a very wide used Advantage
saddle for sale. It has the “English” stirrups leathers set
up on it and the adjustable rigging like on the original Advantages. Contact
me if you are interested and I can get you in touch with the saddle’s
owner.