A suggestion for the sling. Buy a narrow sling, for example a 1" wide leather military style sling. The narrow sling will bite into your shirt or jacket better than the wider sling and thus tend to stay in position better rather than sliding off. The purpose of a wider sling is to distribute weight, to lessen the pressure at the point of contact -- and of course this works -- but the wider sling reduces the bite of the sling into your clothes and makes it easier to slide off. Maybe you can borrow a sling like this from someone to give this suggestion a risk-free trial?