I second (or third) trying a mohair string cinch. If I don't let them get too nasty/dirty and I keep them adjusted correctly, I don't have problems with them galling even the most sensitive horses. Oh, and stay away from roper style cinches and the ones that have leather binding in the center like this
The roper style takes up more room and can make the galls worse if you are already having a fit problem. As for the leather middle, that will gall a horse like nobody's business.
You'll want to stick to this type, a regular straight cinch, with a nylon center...and make sure you keep it centered on the horse.
Also, when I was training for the public, I bought this stuff in bulk. Some may not believe me when I say so, but this actually helps the gall heal without having to give the horse time off. Just slather on a nice layer before the ride, rinse them off and slather on some more after the ride, and all but the very worst galls will be healed in just a few days. It won't sweat off and water won't rinse it off, plus, it keeps the bugs off the wound without having to apply any stinging fly spray to the area. I've used it on myself when I got a scrape or rope burn up at the barn (not something I'd use before a night on the town though as it does have a smell that most would consider unpleasant, not bad
smelling, just not good either
). Gall Salve Weaver Livestock Supply (Health Care - Wound Care)
ETA: Hahaha, I didn't realize I was linking the same salve as Dreamcatcher
. Great minds think alike, it's amazing stuff.