I have a gel pad but it just sticks to the saddle nothing else😂
I used to ride a horse with a similar problem and found that a 3-point breastplate helped to stabilise it
If it's fitted don't bulk up with saddle pads, just use a basic thin saddle pad, otherwise it's like for example your a size 8 shoe and then you put on 3 pairs of socks and suddenly your shoe is very tight!
Maybe a gel and sheepskin combination pad might help
Have you tried a non slip pad?