Looks like a challenge. Good luck! It is the same sort of problem we have with Cowboy. I've always liked the analogy that a saddle fits a horse's back like two spoons together - there is just a place where the curves make it fit, and it will inevitably go there.
But with Cowboy, it is more like a couple of butter knives in a drawer. There just isn't anything the saddle can fit into. Just a broad but short back, hips pushing the saddle forward and nothing on the front end that screams "Stop moving forward!" It is frustrating, and it must be for Cowboy as well. The only time he bucks is when the saddle slips forward onto his should blades. The front lift Wintec pad helps, but I'd like a better solution for Cowboy's sake.