Your best bet is to go with a high quality leather girth.
I have a Schleese girth and its very nice and ergonomically shaped and doesn't interfere with the shoulder.
I have also a Jaguar girth which is less shaped, but works well on horses with less movement.
Look for something that is quite skinny in the shoulder/elbow area, doesn't have a lot of elastic, no neoprene, no cheap or fake sheepskin, and a high quality, thick leather. If you do go for sheepskin, go genuine and make sure it is removable and washable. But remember some horses are sensitive to it.
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