I have the same problem with sheepskin on my horse, except when his skin gets irritated I get to deal with fun fungal infections. As well I find sheepskin to be really bouncy. So I avoid it!!
Ogilvy is memory foam (and cusomizable) but if you really want a pad with lots of shimming, EcoGold does a great memory foam pad with "filler pads". I use the EcoGold Secure pad, because it has just a little wee bit of memory foam, not enough to change my saddle's fit, but enough that my back (and his!) are cushioned.
You can see her wither has a steep slope into a flat back, that's what I'm thinking the tree is not conforming to, that it's made for a horse with a smoother wither slope and a curvy back.
They say money doesn't buy happiness -- well happiness doesn't buy horses!