For a horse with muscle atrophy, or hollows, behind the shoulder blades and prominent withers, this shoulder bridge pad from Tucker in the answer.
Based on Tucker's vast saddle fitting experience, they offer several styles of bridge pads to accommodate some of the most common conformation issues that interfere with proper saddle fit.
Designed to fit under your regular saddle pad, the bridge pads are made with memory foam that fill in the gaps between your horse and saddle which provides a more comfortable fit for your horse.