How to protect my horse's hocks from scs when he gets up from lying down?

My big Quarter Horse has open sores on the outsides of both hocks from scraping them as he gets up from sleeping on the rubber mats in his run-in shed. I don't use shavings, and really do not want to have to deal with bedding. I sweep the rocks and debris out everyday, but it hasn't helped. I have tried fleece-lined hock boots, but no matter how tight I tighten the velcro, they have slipped off by the next morning. I have tried stretchy neoprene men's knee braces, but the same thing. My horse seems to rub them off as he stands up. I tried cut-off tube socks, but cannot stretch them over his large hooves. Any ideas on how I can protect his legs? Thanks so much.